Application Programming Interface

An application programming interface (API) is a defined way for two pieces of software to talk to each other and swap information, without either needing to know how the other works inside. When a travel site shows live flight prices, it is using the airline's API to ask for them.

It is like a restaurant menu and waiter: you order from a fixed list and the kitchen sends out the dish, and you never enter the kitchen yourself. Application programming interfaces matter because they let your tools connect, so your shop, your accounts and your email can share data automatically instead of by hand.