APIs

An API (Application Programming Interface) is a defined way for one piece of software to talk to another — a set of requests it accepts and responses it gives back, without either side needing to know the other's internal details.

Real-world analogy: a restaurant menu is an API. You (the caller) order from a fixed list of choices; the kitchen (the other system) does whatever internal work is needed and returns a plate of food (the response) — you never need to know how the kitchen works inside.

A very common shape today is a web API: your program sends a request to a URL over the internet and gets structured data (often JSON) back.