API (Application Program Interface)An API is an interface that allows an application program to access a special set of prepackaged services. These services may come from the operating system, or may deliver other services. For example, it's not unusual to find networking services of all kinds delivered in the form of one or more protocol-, service-, or transport-specific APIs for developers to use. An API, defined at the source code level, provides a certain level of abstraction from nitty-gritty system-level details, and can sometimes help to ensure the resulting code's portability.
An API usually provides a connection between a higher level programming language and lower level system and other services (particularly networking). Such services are often developed without consideration for the calling conventions supported by the higher-level languages typically used by application developers. In such a case, the main job of an API may be to simply re-order and interpret parameter lists from one format to another, allowing the lower-level services to interact with an application program.