What is an API?
An Application Programming Interface, a contract that lets one program request actions or data from another in a stable, documented way.
An API defines what one piece of software can ask another to do, and how to ask. In modern web development, "API" usually means a web API: a service exposes HTTP endpoints, you send requests with parameters, the service returns JSON. The contract is the documented endpoint list, the parameters each accepts, and the response shapes you can depend on.
APIs solve the integration problem. Instead of every CRM building its own logo lookup, every fintech writing its own merchant categorizer, and every AI app implementing its own scraper, each capability lives behind a stable API and any caller plugs in. The economic story of the cloud is largely a story of APIs replacing in-house implementations.
For developers, the practical API skills are: reading the docs to understand auth and rate limits, building a typed client that handles pagination and errors well, and resisting the temptation to wrap every API in a leaky abstraction. Most production bugs at the integration boundary come from clients that pretended their API was something it wasn't.
In the wild
- Stripe's payment API,
POST /v1/charges - Context.dev's Brand API,
POST /v1/brand/retrievewithtype: "by_domain" - Twilio's SMS API,
POST /Messages.json