What is an API and how does it work?

Think of an API as a contract: it specifies what requests you can send, what data you need to include, and what response you'll get back — without exposing the internal code that makes it happen.

A simple example

A weather app on your phone doesn't run its own network of weather sensors. Instead, it sends a request to a weather service's API — for example, asking for the forecast for a given city — and the API responds with structured data (temperature, humidity, conditions) that the app then displays.

Common types of APIs

• REST APIs — the most common web API style, using standard HTTP methods (GET, POST, PUT, DELETE)
• GraphQL APIs — let the client specify exactly which fields of data it wants in one request
• SOAP APIs — an older, more rigid XML-based standard still used in some enterprise systems
• Library/OS APIs — not networked at all; they define how one piece of code can call functions in another (e.g. a programming language's standard library)

APIs are what let separate companies' software integrate — a payment API lets an online store accept credit cards without building its own banking infrastructure, for example.