APIs exist for most enterprise software, and they are essential in operating systems, which control most of a computer's basic functions. You often have to rely on others to perform functions that you may not be able or permitted to do by yourself, such as opening a bank safety deposit box. Similarly, virtually all software has to request other software to do some things for it. Almost every application depends on the APIs of the underlying operating system to perform such basic functions as accessing the file system. In essence, a program's API defines the proper way for a developer to request services from that program.
Developers can make requests by including calls in the code of their applications.
The syntax is described in the documentation of the application being called. By providing a means for requesting program services, an API is said to grant access to or open an application. The API for all computing purposes is how you open the blinds and the doors and exchange information. SAP AG's enterprise applications include APIs, called BAPI, that offer other applications access to business data. When an industry settles on a data standard, a common API allowing access to applications that process that data often follows, Walker says.