An application program interface (API) is a set of routines, protocols, and tools for building software applications. Basically, an API specifies how software components should interact.
There are many different types of APIs for operating systems, applications or websites. Windows, for example, has many API sets that are used by system hardware and applications — when you copy and paste text from one application to another, it is the API that allows that to work.
1- Google Maps API.
2- YouTube APIs.
3- Twitter APIs.
4- Amazon Product Advertising API.
Your phone’s data is never fully exposed to the server, and likewise the server is never fully exposed to your phone. Instead, each communicates with small packets of data, sharing only that which is necessary—like ordering takeout. You tell the restaurant what you would like to eat, they tell you what they need in return and then, in the end, you get your meal.