API - application program interface

API, an abbreviation of application program interface, is a set ofroutines, protocols, and tools for building software applications. The API specifies how software components should interact and APIs are used when programming graphical user interface (GUI) components.  A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

Types of APIs

There are many different types of APIs for operating systems, applications or for websites. Windows, for example, has many API sets that are used by system hardware and applications — when you copy and paste text from one application to another, it is the API that allows that to work.

Most operating environments, such as MS-Windows, provide an API so that programmers can write applications consistent with the operating environment. Today, APIs are also specified by websites. For example, Amazon or eBay APIs allow developers to use the existing retail infrastructure to create specialized web stores. Third-party software developers also use Web APIs to create software solutions for end-users.