What Is API?

How API Works

APIs power almost every modern app you use. When you check the weather on your phone, the app calls a weather API. When you log in with Google, that's the OAuth API. When Stripe processes a payment, that's their REST API. APIs are the glue between services.

There are several API styles. REST uses HTTP verbs and URLs. GraphQL lets clients request exactly the fields they need. gRPC uses protocol buffers for high-performance streaming. WebSocket APIs enable real-time bidirectional communication.

Most APIs use JSON for data exchange, authenticate with API keys or OAuth tokens, and return HTTP status codes (200 OK, 404 Not Found, 500 Server Error). Good APIs have versioning, rate limits, and clear documentation.

The API economy is massive — Stripe, Twilio, SendGrid, and thousands of SaaS companies are essentially API-first businesses. If you build software, you will consume and likely build APIs.

Why Developers Use API

Every modern application relies on APIs. Frontend apps call backend APIs for data. Microservices communicate through internal APIs. Third-party integrations (payments, email, maps, auth) all work through APIs. Even your IDE's extensions use APIs.

Key Concepts

• Endpoint — A specific URL that accepts requests — like GET /api/users returns a list of users
• HTTP Methods — GET reads data, POST creates, PUT updates, DELETE removes — the verbs of REST APIs
• Request/Response — The client sends a request with headers and optional body; the server returns a status code and data
• Authentication — APIs use keys, tokens, or OAuth to verify who's calling — never ship an API without auth
• Rate Limiting — APIs cap how many requests you can make per time window to prevent abuse and ensure fair usage
• Status Codes — 200 means success, 400 means bad request, 401 unauthorized, 404 not found, 500 server error

Fetching Data from an API

// Fetch users from a REST API
const response = await fetch('https://api.example.com/users', {
  headers: {
    'Authorization': 'Bearer YOUR_API_KEY',
    'Content-Type': 'application/json'
  }
});

const users = await response.json();
console.log(`Found ${users.length} users`);

Frequently Asked Questions