What is an API?

An API, short for application programming interface, is a set of rules that lets different software applications talk to each other and exchange data. One program asks in an agreed format and the other answers in an agreed format, and neither needs to know how the other works inside. Almost everything online that talks to anything else is doing it through an API.

An API is a published way for one piece of software to ask another to do something: send a request in this format, and get a response in that format. A weather service might promise that if you send a city name, it returns today's forecast. The app asking never needs to know how that forecast was produced, only how to ask for it. That separation is the whole point, because it lets both sides change independently. The weather service can rebuild its entire system without breaking every app that uses it, as long as the API stays the same. The owner also decides which requests are allowed, who can make them, and how often. If those rules change in incompatible ways then old apps stop working, so companies usually release a new version rather than altering the old one. Modern AI systems lean on APIs constantly, whether they are checking the weather, reading your calendar or booking a flight.

A restaurant menu. It lists exactly what you can order and what arrives if you do, and you never see the kitchen, meet the chef or learn the recipe. The kitchen can replace its ovens or change suppliers without telling you, and your order still works, because the menu is the agreement. Ask for something not on the menu and the answer is simply no.