API

An application programming interface (API) is a set of routines, data structures, object classes and/or protocols provided by libraries and/or operating system services in order to support the building of applications.[1] An API may be:

Language-dependent; that is, available only in a particular programming language, using the particular syntax and elements of the programming language to make the API convenient to use in this particular context.

Language-independent; that is, written in a way that means it can be called from several programming languages (typically an assembly/C-level interface). This is a desired feature for a service-style API that is not bound to a particular process or system and is available as a remote procedure call.

There are several kinds of API publishing policies often encountered, including the following:

Companies do not release API information to anybody outside of the company.
Companies protect information on their APIs from the general public. For example, Sony used to make its official PlayStation 2 API available only to licensed PlayStation developers. This enabled Sony to control who wrote PlayStation 2 games. Such control can have quality control benefits and potential license revenue.
Companies make their APIs freely available. For example, Microsoft makes the Microsoft Windows API public and Apple releases its APIs Carbon and Cocoa so that software can be written for their platform.

[edit] API examples
Below are listed some well-known APIs.

The PC BIOS call interface
Comedi Data Acquisition API
Single UNIX Specification (SUS)
Windows API
The various Java Platform Edition APIs (Micro, Standard, Enterprise)
ASPI for SCSI device interfacing
Carbon and Cocoa for the Macintosh OS
iPhone API
OpenGL cross-platform 3D graphics API
DirectX for Microsoft Windows
Simple DirectMedia Layer (SDL)
Google Maps API
MediaWiki API
YouTube API
PayPal Payment Pro
Facebook API (Facebook)