Application Programming Interface, or API, 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 these blocks together.
Most operating environments, such as Microsoft Windows, provide an API, so that programmers can write applications consistent with the Operating Environment. Although APIs are designed for programmers, they are ultimately good for users because they guarantee that all programs using a common API will have similar interfaces. This makes it easier for users to learn new programs.