Application programming interface

An Application Programming Interface (API) is a set of functions, procedures, methods or classes used by computer programs to request services from the operating system, software libraries or any other service providers running on the computer. A computer programmer uses the API to make
application programs.
An API works by communicating with and exchanging data with other systems, acting as a messenger between the user and the system to retrieve the necessary data the user is requesting from the system. 
Main types of web APIs:
Types of API include web services API like the Twitter API, which allows programs to use the API to receive updates on tweets.