What is an API in Programming?

Every time you check the weather, pay online, or log in with Google, an API is doing the work behind the scenes.

APIs, or application programming interfaces, are the connective tissue of modern software. They let systems talk to each other, share data, and trigger actions, without either side exposing its internal code.

Nearly 90% of developers use APIs in some capacity (Slashdata). And yet, for many people, the concept stays fuzzy.

This guide breaks down exactly what an API is, how the request-response cycle works, what REST, SOAP, and GraphQL actually mean, and why APIs matter far beyond the developer world.

What is an API?

An API, or Application Programming Interface, is a set of rules and protocols that allows two software systems to communicate with each other. One system sends a request. The other processes it and returns a response.

APIs sit between the client and the server. The client makes the request, the server handles it, and the API defines exactly how that exchange happens. Think of the backend of any web app: it exposes API endpoints that the frontend calls to fetch or send data.

According to Postman’s 2025 State of the API Report, 82% of organizations have adopted some level of an API-first approach, up from 74% in 2024. APIs are no longer a developer tool. They are foundational infrastructure.

Nearly 90% of developers use APIs in some capacity, according to Slashdata’s Developer Economics Survey. Over 83% of all web traffic passes through some form of API (Akamai).

What does an API actually do?

An API exposes specific functions of a software system without exposing the internal code behind them. You get access to what the system can do, not how it does it.

3 things every API does:

• Accepts a structured request from a client application
• Passes that request to the appropriate backend service
• Returns a structured response in a defined format, usually JSON or XML

When you tap “Pay with Stripe” on a checkout page, your browser sends an API call to Stripe’s payment service. Stripe processes it and returns a success or failure response. You never see Stripe’s internal payment logic.

Why the term “interface” matters

An interface is a contract. It defines what inputs are accepted and what outputs will be returned, independent of how either side is built.

This means the frontend can be written in JavaScript and the backend in Python, and they communicate cleanly through the API contract. Neither side needs to know how the other works internally.

That separation is what makes modern software scalable. It is also why APIs form the backbone of microservices architecture, third-party integrations, and mobile applications.

How Does an API Work?

Every API interaction follows the same pattern: a client sends a request to a server endpoint, the server processes the request, and a response comes back. This is the request-response cycle.

The most common transport layer is HTTP, the same protocol browsers use to load web pages. REST APIs run on top of HTTP, which is a big part of why REST became the dominant API architecture.

What happens during an API call?

Postman’s 2025 report found that 93% of API teams struggle with collaboration, often because developers don’t fully understand what happens inside each call. Here is the actual sequence:

1. The client constructs a request with a method (GET, POST, PUT, DELETE), an endpoint URL, optional headers, and an optional body payload
2. The request travels over the network to the server
3. The server authenticates the request, validates the input, and runs the relevant logic
4. The server returns a response with an HTTP status code and a payload

Status codes tell the client what happened. 200 means success. 401 means unauthorized. 404 means the endpoint doesn’t exist. 500 means something failed on the server side.

Request structure

The 4 components of an API request:

• Endpoint: the URL path that maps to a specific resource, such as /users/42
• Method: the HTTP verb that defines the action, GET to read, POST to create, PUT to update, DELETE to remove
• Headers: metadata about the request, including authentication tokens, content type, and accepted response formats
• Body: the data payload sent with POST or PUT requests, usually formatted as JSON

Response structure

The server response mirrors the request structure. It carries a status code, response headers, and a body. Most modern APIs return JSON-formatted payloads because JSON is lightweight and easy for both humans and machines to parse.

Ajax is one of the most common ways browsers handle API responses on the client side. It allows a page to update data without a full page reload by processing the JSON response asynchronously.

Latency in the request-response cycle comes from 3 sources: network round-trip time, server processing time, and payload size. Optimizing any one of these improves API performance.

What Are the Main Types of APIs?

There are 4 primary API architectures in active use: REST, SOAP, GraphQL, and gRPC. Each solves a different problem and fits a different context.

REST powers 83% of all web services, according to RapidAPI’s developer data. The other 3 fill specific gaps where REST falls short.

REST vs SOAP vs GraphQL vs gRPC

REST (Representational State Transfer) was defined by Roy Fielding in 2000. It uses standard HTTP methods and resource-based URLs. Simple, stateless, and well-supported by every tool and language.

SOAP (Simple Object Access Protocol) predates REST and uses strict XML contracts defined by a WSDL file. Verbose and complex, but still the standard in financial services and healthcare where guaranteed delivery and message-level security matter.

GraphQL was developed internally at Meta in 2012 and released publicly in 2015. A 2024 Hygraph survey found that 61% of organizations use GraphQL in production. It uses a single endpoint and lets clients request exactly the fields they need. GitHub’s GraphQL API reduced their mobile app data usage by 60% after switching from REST.

gRPC was built by Google and uses HTTP/2 with binary Protocol Buffer serialization. Roughly 7x faster than REST for internal service-to-service calls due to binary encoding and multiplexed connections. Common in high-throughput microservices at companies like Netflix and Google.

What is an API Endpoint?

An API endpoint is a specific URL path that maps to a resource or action on a server. It is the address a client sends a request to when it wants to read, create, update, or delete data.

Endpoints are the most visible part of an API. Every function the API exposes has a dedicated endpoint.

Endpoint structure

A typical REST endpoint is made up of a base URL plus a resource path:

https://api.example.com/v1/users/42

Breaking that down:

• Base URL: https://api.example.com – the root address of the API server
• Version: /v1 – signals which version of the API is being called
• Resource: /users – the collection being accessed
• Identifier: /42 – the specific record within that collection

HTTP methods and endpoints

The same endpoint path can serve multiple operations depending on the HTTP method used. This is one of REST’s core design principles: resources are nouns, methods are verbs.

A missing or misspelled endpoint returns a 404 status code. An endpoint that exists but rejects the request due to permissions returns 403. These HTTP status codes are the primary way APIs communicate errors back to the client.

What is an API Key and How Does API Authentication Work?

API authentication controls who can access an API and what they are allowed to do. Without it, any client could send unlimited requests, scrape data, or trigger operations without accountability.

There are 3 primary authentication mechanisms in use: API keys, OAuth 2.0, and JWT tokens.

API keys

An API key is a unique string token passed in a request header or query parameter to identify the calling application.

Simple to set up. Widely used for server-to-server calls where the key can be stored securely. The main weakness: if the key is exposed, anyone can use it. No granular permissions. No expiry by default.

Stripe processes over 500 million API requests daily, most authenticated via API keys at the application level (Stripe, 2025). They pair key authentication with rate limiting and IP allow-listing to reduce exposure risk.

OAuth 2.0

OAuth 2.0 is a delegated authorization framework. It lets a user grant a third-party application limited access to their account without sharing their password.

When you click “Login with Google” on a site, OAuth 2.0 handles the handshake. Google issues an access token to the third-party app. That token grants specific permissions for a limited time. The app never sees your Google password.

Used by Google, GitHub, Facebook, and most major platforms for cross-application authorization. More complex to implement than API keys but significantly more secure for user-facing applications.

JWT (JSON Web Tokens)

JWTs are self-contained, signed tokens that carry user identity and permission claims inside the token itself. The server doesn’t need to look up a session in a database. It verifies the signature and reads the claims directly.

3 parts of a JWT: a header (algorithm used), a payload (claims like user ID and expiry), and a signature (verifies the token hasn’t been tampered with).

JWTs are stateless, which aligns well with REST’s stateless constraint. Common in single-page applications and progressive web apps that need to authenticate API calls without server-side session storage.

What is an API in Web Development?

In web development, APIs fall into 2 categories: browser-native Web APIs built into the browser itself, and external third-party APIs accessed over HTTP. Both are used constantly. Most developers work with both daily without thinking about the distinction.

Postman’s 2024 State of API Report found that AI-related API traffic increased by 73% in a single year, reflecting how quickly external API consumption has grown beyond traditional integrations.

Browser Web APIs

The browser exposes dozens of native APIs through JavaScript. These are not HTTP-based. They are interfaces to browser functionality, available without any external service.

• Fetch API: makes HTTP requests from the browser to external servers
• DOM API: reads and modifies the structure of an HTML document
• Web Storage API: provides localStorage and sessionStorage for client-side data persistence
• Canvas API: draws graphics programmatically in a browser canvas element
• Geolocation API: retrieves the user’s physical location with their permission

Third-party API integrations

External APIs are services exposed by other companies that developers call over HTTP. They handle functionality that would take months to build from scratch.

Common third-party APIs in web projects:

• Stripe: payment processing, subscriptions, and invoicing
• Twilio: SMS, voice calls, and WhatsApp messaging
• SendGrid: transactional and marketing email delivery
• Google Maps: maps, geocoding, and directions embedded in applications

Using these APIs instead of building equivalent systems reduces development time dramatically. It also shifts infrastructure responsibility, including uptime, scaling, and security, to the API provider.

Frontend vs backend API usage

The user interface layer calls APIs from the browser using JavaScript’s Fetch API or a library like Axios. These are visible in browser developer tools under the Network tab.

The backend makes its own server-to-server API calls. A Node.js server calling Stripe’s API to charge a card never touches the browser. Those calls happen entirely on the server side, invisible to the end user.

CSS handles visual presentation, but it has no role in API communication. The division of responsibilities is clean: HTML structures the page, CSS styles it, JavaScript calls the APIs, and the backend handles server-side logic.

What is a REST API?

A REST API is an application programming interface that follows the constraints of Representational State Transfer, an architectural style defined by Roy Fielding in his 2000 doctoral dissertation. REST is not a protocol. It is a set of design principles applied on top of HTTP.

83% of all web services run on REST (RapidAPI). It is the default choice for public APIs because it uses the same HTTP methods browsers already understand.

The 6 REST constraints

Fielding defined 6 constraints that an API must follow to be considered RESTful. Most developers know 2 or 3 of them. All 6 matter.

• Stateless: each request must contain all information needed to process it. The server stores no client session between requests.
• Client-server separation: the client and server evolve independently. The API contract is the only shared boundary.
• Cacheable: responses must declare whether they can be cached. Proper caching reduces load on the server and improves client performance.
• Uniform interface: consistent use of HTTP methods and resource-based URLs across all endpoints.
• Layered system: the client cannot tell whether it is connected directly to the server or through an intermediary like an API gateway or load balancer.
• Code on demand (optional): servers can send executable code to clients, such as JavaScript. The only optional constraint.

What makes an API RESTful?

The label “RESTful” is used loosely. A lot of APIs called RESTful violate at least one of Fielding’s constraints.

Common violations:

• Using POST for all actions instead of matching methods to operations (GET, PUT, DELETE)
• Storing session state on the server between requests
• Returning HTML or mixed content instead of clean JSON payloads

Statelessness is the constraint that trips up the most implementations. Once a server starts storing client state between requests, the scalability benefits of REST start to break down. Each server instance needs to know about the session, which complicates horizontal scaling.

GitHub’s public API is a widely studied example of a well-designed RESTful API. It uses consistent resource naming, proper HTTP methods, clear status codes, and pagination headers. Their migration to a GraphQL API for more complex data queries shows that even well-built REST APIs have limits when client data needs become highly variable.

What is an API in the Context of Microservices?

In a microservices architecture, APIs are the communication contracts between independent services. Each service does one job, and APIs define exactly how services talk to each other without sharing internal code or databases.

Netflix operates over 1,000 loosely coupled microservices that handle more than 2 billion API requests daily (Openapi.com). Every function, from authentication to recommendations to video playback, runs as a separate service and communicates through APIs.

How APIs connect microservices

Service-to-service communication patterns:

• Synchronous calls: one service sends an HTTP API request and waits for a response before continuing
• Asynchronous messaging: services publish events to a message queue (like Apache Kafka) and other services consume them independently
• gRPC: used for high-speed internal calls between services where binary performance matters

Netflix migrated from a monolithic application to microservices after a 3-day outage in 2008 caused by database corruption. The migration took 7 years and completed in 2016. Their architecture now handles hundreds of millions of streaming requests without a single point of failure.

API contracts in distributed systems

An API contract defines the exact inputs and outputs each service expects. Break the contract, and you break every service depending on it.

This is why API versioning exists. When a service updates its response structure, the old version stays active while consumers migrate at their own pace. Most large systems run at least 2 active API versions simultaneously.

The API gateway sits in front of all microservices, handling routing, authentication, and rate limiting in one layer. Without it, every client would need to know the address of every service, which becomes unmanageable fast.

Service mesh vs API gateway

API gateway: manages north-south traffic, meaning requests from external clients into the system.

Service mesh: manages east-west traffic, meaning service-to-service communication inside the system.

Companies like Netflix use both. The API gateway handles what comes in. The service mesh handles what happens internally.

What is an API Gateway?

An API gateway is a server that acts as the single entry point between external clients and internal backend services. It handles routing, authentication, rate limiting, logging, and load balancing before a request ever reaches a backend service.

The global API gateway market was valued at $4.3 billion in 2024 and is projected to reach $20.2 billion by 2033, growing at a CAGR of 20.8% (Global Growth Insights).

What an API gateway does

63% of enterprises integrate API gateways into their CI/CD pipelines to manage application speed, scalability, and monitoring (Global Growth Insights, 2024).

Core functions of an API gateway:

• Request routing: directs each incoming request to the correct backend service
• Authentication: verifies API keys, JWT tokens, or OAuth credentials before forwarding
• Rate limiting: caps the number of requests a client can make in a defined time window
• Load balancing: distributes traffic across multiple service instances
• Logging and analytics: records every request for monitoring and debugging

API gateway tools

Kong released a developer-first gateway in late 2023 with dynamic plugin orchestration, achieving 31% faster developer onboarding for API-centric applications (Global Growth Insights).

API gateway vs reverse proxy

A reverse proxy forwards requests and handles TLS termination. It does not understand API-specific concepts like route versioning, token scopes, or per-consumer rate limits.

The distinction matters in production. An NGINX reverse proxy can sit in front of an API gateway, but it cannot replace one. Organizations running microservices at scale need both, with the reverse proxy handling raw traffic and the gateway handling API-level logic.

What is a Public API vs a Private API?

APIs are classified by who can access them. 3 access categories exist: public, private, and partner. Each carries different design requirements, security considerations, and business implications.

Gartner found that 71% of digital businesses consume APIs created by third parties, which means most organizations are already using public APIs whether they have a formal API strategy or not (Platformable, 2024).

Public APIs

Available to any external developer, with or without registration. Usually rate-limited. Sometimes monetized.

The Google Maps API powers location features in tens of thousands of apps. Twitter/X’s API enables third-party clients, analytics tools, and bots. These APIs are products in their own right, with dedicated developer portals, versioning policies, and pricing tiers.

A 2024 Salt Security report found API counts inside organizations grew by 167% in a single year, driven partly by the explosion of publicly consumed third-party APIs.

Private APIs

Used internally only. Not exposed to the outside world. The Salesforce backend, for example, uses thousands of internal APIs to connect its CRM, billing, marketing, and analytics services.

Private APIs still need documentation, versioning, and access controls. Teams inside an organization are API consumers too, and a poorly documented internal API creates the same friction as a bad public one.

Partner APIs

Shared only with specific business partners under a formal agreement. Not listed in public directories. Requires authenticated access and usually includes usage monitoring tied to the partner relationship.

MuleSoft’s 2025 Connectivity Report found that 40% of enterprise revenue is now generated from API-related implementations, up from 25% in 2018. Partner APIs drive a significant portion of that number, since they power integrations between businesses that generate direct transactional value.

What is API Documentation?

API documentation describes every aspect of an API: its endpoints, accepted inputs, expected outputs, error codes, authentication methods, and rate limits. Without documentation, an API cannot be adopted.

SmartBear’s State of Software Quality survey found that 70% of organizations cite API quality as a top priority, yet 43% of those same respondents rated their own documentation as poor and in need of improvement.

What complete API documentation contains

62% of API professionals use a dedicated design and documentation tool like Swagger UI or Postman, according to SmartBear’s 2024 report.

The 6 components every API reference needs:

• Authentication method and token format
• Full list of endpoints with HTTP methods
• Request parameters, types, and required vs optional fields
• Response structure and example payloads
• Complete error code list with descriptions
• Rate limit rules and retry guidance

OpenAPI Specification and tooling

The OpenAPI Specification (formerly Swagger) is the standard format for describing REST APIs in a machine-readable YAML or JSON file. Tools like Swagger UI and Redoc generate interactive documentation directly from an OpenAPI spec file.

Swagger is adopted by 28% of API teams for design, documentation, and developer collaboration (SQ Magazine, 2025). OpenAPI Generator is used by an additional 20%, automating code generation from the same spec.

Postman is the most widely used environment for API testing, at 80%, and doubles as a documentation tool for teams that want browsable request examples alongside their reference docs (Nordic APIs, SmartBear 2023).

Why documentation quality drives adoption

64% of API professionals cite limited time as the top obstacle to keeping documentation current (SmartBear). Another 47% deal with docs that have drifted out of sync with the actual API implementation.

When documentation lags behind the API, developers waste time debugging incorrect examples or making requests that no longer match the current schema. That friction directly reduces adoption rates for both internal and external APIs.

What Are API Rate Limits?

An API rate limit is a cap on the number of requests a client can send within a defined time window. Exceeding the limit triggers an HTTP 429 “Too Many Requests” response.

Rate limiting protects servers from overload, prevents abuse, and enforces fair usage across all consumers sharing the same API infrastructure.

How rate limits are structured

Rate limits are set as request counts per time unit. Common formats:

• 1,000 requests per hour
• 100 requests per minute
• 10 requests per second

The time window resets on a fixed schedule (fixed window) or rolls continuously based on the last request (sliding window). Fixed windows are simpler to implement. Sliding windows are more accurate for burst traffic control.

Handling rate limit responses

Salt Security’s 2024 API security report found that lack of rate limiting ranks among the top 4 most exploited API vulnerabilities, tied with broken user authentication at 12% of all attack attempts.

Well-implemented rate limit responses include a Retry-After header that tells the client exactly how many seconds to wait before retrying. Clients that ignore this header and retry immediately compound the problem and may trigger longer lockouts.

3 strategies for handling rate limits in client code:

• Exponential backoff: double the wait time after each failed retry, starting at 1 second
• Request queuing: buffer outgoing requests and send them at a controlled pace below the limit
• Response caching: store API responses locally and serve cached data instead of hitting the API again for repeated identical requests

Rate limits as a security layer

Rate limiting is not just about performance. It is a first-line defense against credential stuffing, brute force attacks, and data scraping.

An attacker cycling through user IDs to scrape account data will generate abnormally high request volumes from a single token. A properly configured rate limit catches that pattern and cuts off access before significant data is exposed.

What is an API in Business and Non-Technical Terms?

In business terms, an API is a connector that lets software products share capabilities without rebuilding them from scratch. When a checkout page shows a “Pay with PayPal” button, that button runs on PayPal’s API. The merchant never built a payment system. They plugged into one.

MuleSoft’s 2025 Connectivity Report found that IT leaders now estimate 40% of their company’s revenue is generated from API-related implementations. That number rises to 45% at organizations that have adopted AI agents.

The API economy

APIs have created an entirely separate layer of the technology economy. Companies expose their core functionality as APIs and charge other businesses to use it.

Examples non-technical people recognize daily:

• “Login with Google” on any website runs on Google’s OAuth API
• Delivery time estimates in food apps come from Google Maps or HERE APIs
• Fraud detection in banking apps often runs on third-party risk scoring APIs
• Weather widgets in any app pull data from a meteorological service API

Stripe’s payment API processes over 500 million API requests every day, roughly 15 billion per month (Stripe, 2025). Stripe is not just a payment processor. It is an API product that other businesses pay to use.

Build vs integrate

Building a payments system from scratch costs millions and takes years. Integrating Stripe’s API costs a few hundred dollars in developer time and goes live in days. The same logic applies to email, maps, identity verification, translation, and dozens of other functions.

That cost difference is what drives API adoption across every industry. The API management market is projected to grow from $4 billion in 2025 to $19.3 billion by 2034, at a CAGR of 19.05% (Market Reports World). The market exists because the build-vs-integrate math almost always favors integration.

APIs and the user experience layer

Most user experience improvements in modern apps come from API integrations running invisibly in the background. Autofill addresses, real-time currency conversion, instant payment confirmation. None of those are built by the app developer. They are all API calls.

The web accessibility layer also benefits. Third-party APIs now provide real-time translation, screen reader content optimization, and alternative text generation, functions that would be prohibitively expensive for most development teams to build internally.

FAQ on APIs

What does API stand for?

API stands for Application Programming Interface. It is a set of rules that allows two software systems to communicate. One system sends a request, the other processes it and returns a response.

What is an API used for?

APIs let applications share data and functionality without exposing internal code. Common uses include payment processing, login authentication, maps, and messaging. Stripe, Google Maps, and Twilio are all API-based services.

What is a REST API?

A REST API follows the Representational State Transfer constraints defined by Roy Fielding in 2000. It uses standard HTTP methods, resource-based URLs, and stateless requests. REST powers 83% of all web services today.

What is the difference between an API and a web service?

All web services are APIs, but not all APIs are web services. A web service specifically communicates over a network using HTTP. APIs can also operate locally between software components on the same machine.

What is an API endpoint?

An API endpoint is a specific URL that maps to a resource or action on a server. For example, GET /users/42 retrieves the user with ID 42. Each endpoint corresponds to one defined operation.

What is an API key?

An API key is a unique token passed in a request header to identify the calling application. It controls access and enables rate limiting. If exposed publicly, anyone can use it to make requests on your behalf.

What is the difference between REST and GraphQL?

REST uses multiple endpoints, each returning a fixed data structure. GraphQL uses a single endpoint and lets clients request exactly the fields they need. GitHub’s GraphQL API reduced mobile data usage by 60% versus REST.

What is an API gateway?

An API gateway is a server that sits between clients and backend services. It handles routing, authentication, rate limiting, and logging in one layer. Tools like AWS API Gateway, Kong, and Apigee are widely used examples.

What is API rate limiting?

Rate limiting caps the number of requests a client can make within a set time window. Exceeding the limit returns an HTTP 429 status code. It protects servers from overload and prevents abuse or data scraping.

What is the difference between a public API and a private API?

A public API is accessible to any external developer, often through a developer portal. A private API is used internally within one organization. Partner APIs sit between the two, shared only with specific business partners under agreement.

Conclusion

This conclusion is for an article presenting what is an API, from its core request-response cycle to authentication, rate limiting, and the broader API economy.

REST remains the dominant architecture, but GraphQL, gRPC, and SOAP each fill specific gaps depending on your data needs and system design.

API documentation, versioning, and gateway management are not optional at scale. They are what separates a reliable integration from a fragile one.

Whether you are consuming third-party services like Stripe or building internal microservices, the same principles apply: clear contracts, proper authentication, and controlled access.

The OpenAPI Specification, OAuth 2.0, and JSON Web Tokens are the standards worth knowing. Start there, and the rest follows naturally.