API stands for Application Programming Interface.

Application programming interface or APIs act as an interface between two different programs.

API is like a user interface that helps a user communicate with a computer.

API can be language dependent or language independent. Language dependent API can be used only by using the particular syntax of a programming language in which the API is implemented.

Language independent API is a generic service provider and can be called by any language. Generally service oriented API are language independent. 

API provides specific rules and instructions for a software program that would enable that program to communicate and utilize the services that are being offered by another software or program. In this scenario the API can be considered as a bridge that links two different programs together.

Application programming Interface is used by operating systems, applications, software and enterprise systems. API provides a mechanism to define the vocabularies of a program and define how a function from the implementer can be called by a host.

Operating system API are also used by programming languages, for example the COUT>> statement in C++ programming languages uses the Windows API for printing text on the screen. Similarly the JAVA programming languages uses various APIs for database connectivity and providing user interface.

API can be defined for specific programs such as Google maps application programming interface, a service that enable Google maps display and simulation on user’s computer and the JAVA API that is used to provide services for XML.  API are also released by various software companies to help developers build various applications using their services.