API (Application Programming Interface)

An API (Application Programming Interface) is a contract that describes how one program can ask another program to do something, including which requests are allowed and what the responses look like. The caller needs to know only the contract, not how the other side is built.

Why it matters

APIs let teams and companies connect systems: a shop talks to a payment provider, a mobile app talks to its server, a monitoring tool talks to your alerting channel. A stable API lets either side change its internals without breaking the other. When an API is down or slow, everything built on top of it fails too, which is why API endpoints are worth monitoring separately from your website.

Example

GET /v1/users/42 HTTP/1.1
Host: api.example.com
Accept: application/json

HTTP/1.1 200 OK
{"id": 42, "name": "Asha"}

The client asks for user 42 and the server answers with JSON. Web APIs like this one usually run over HTTP, but libraries and operating systems expose APIs too.

Common mistakes

Treating a 200 response as proof the API works; the body can still contain an error message.
Changing a response field name without versioning, which breaks existing clients.
Skipping authentication and rate limiting on public endpoints.

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