API, an abbreviation of application program interface, is a set of routines, protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together. The API allows programmers to use predefined functions to interact with a service or an application. (Source: WebOpedia)
In computer programming, an application programming interface (API) is a set of routines, protocols, and tools for building software applications. An API expresses a software component in terms of its operations, inputs, outputs, and underlying types, defining functionalities that are independent of their respective implementations, which allows definitions and implementations to vary without compromising the interface. A good API makes it easier to develop a program by providing all the building blocks, which are then put together by the programmer.
An API may be for a web based system, operating system, or database system, and it provides facilities to develop applications for that system using a given programming language. As an example, a programmer who develops apps for Android may use an Android API to interact with hardware, like the front camera of an Android-based device.
In addition to accessing databases or computer hardware like hard disk drives or video cards, an API can ease the work of programming GUI components. For example, an API can facilitate integration of new features into existing applications (a so-called "plug-in API"). An API can also assist otherwise distinct applications with sharing data, which can help to integrate and enhance the functionalities of the applications.
APIs often come in the form of a library that includes specifications for routines, data structures, object classes, and variables. In other cases, notably SOAP and REST services, an API is simply a specification of remote calls exposed to the API consumers.
An API specification can take many forms, including an International Standard, such as POSIX, vendor documentation, such as the Microsoft Windows API, or the libraries of a programming language, e.g. the Standard Template Library in C++ or the Java APIs.
An API differs from an application binary interface (ABI) in that an API is source code-based while an ABI is a binary interface. For instance POSIX is an API, while the Linux Standard Base provides an ABI.