API - Application Programming Interface - Allows the application to communicate with each other (e.g. your program with the operating system). It is defined on the source code level (e.g. library for a given programming language). The API's task is to provide appropriate specifications of subroutines (e.g. functions in the library), data structures (e.g. what a given array contains), object classes (define an object in programming j.) and required communication protocols (e.g. http).

For peasant's sake we can say that API allows you to use functionalities provided by another application in your application. An example is the Windows API, which allows you to create applications that take advantage of the capabilities that Windows offers (e.g. in C++ you can open a window using the function defined by the API). When you've been playing games the name DirectX or OpenGL has probably bumped into your ears. These are libraries that allow you to create 2D and 3D applications in programming languages, and they already communicate with your graphics card as standard. This is a great convenience, because APIs like DirectX and OpenGL allow you to write programs for different graphics cards in the same way (manufacturers need to adapt).

API is often used in web applications. A simple example can be downloading tweets from twitter using Twitter API. In your program, you write an HTTP query to the appropriate address and receive the desired tweets from that person.

The API is generally great because it allows you to unify problem solving so that communities can emerge that will improve the circle that has already been created and help others who want to understand how the circle works.

API 代表什么？