What Is an API? Explained for Beginners

An API (Application Programming Interface) is a defined set of rules that lets one piece of software request functionality or data from another, without either side needing to know the other's internal implementation details.

Common causes

• Software systems need to interoperate — a mobile app needs data from a server, one internal service needs data from another — without each side depending on the other's exact source code

How to fix it

• Think of a web API as a menu: it lists the requests (endpoints) you're allowed to make and what you'll get back, without revealing the kitchen (the server's internal code)
• A well-designed API hides implementation details behind a stable contract, so the internals can change freely as long as the API's inputs and outputs stay consistent
• Common API styles include REST (resource-based over HTTP), GraphQL (client-specified queries), and RPC (calling remote functions directly)

Example

GET https://api.example.com/weather?city=London

{"temperature": 14, "condition": "cloudy"}

FAQ

Is an API always over the internet?

No — an API can be a local library's function signatures, an operating system's system calls, or a network API over HTTP. 'Web API' specifically refers to the network-accessible kind.