A collection of programming routines and functions that an application can use to access low-level machine services. Also a set of calling conventions in a programming language that specifies how such a service is invoked through an application. Application programming interfaces (APIs) let C and assembly language routines interact with services and programming tools.
When you write applications for a high-level operating system like Microsoft Windows, you use standard Windows APIs to access standard operating system and networking services and functions. One application can then issue an API call to another application in order to execute that API function. Details of APIs are primarily of interest to developers.
Windows operating systems provide predefined sets of APIs for various purposes, such as Telephony Application Programming Interface (TAPI) for accessing functions related to making voice, data, or fax calls; Messaging Application Programming Interface (MAPI) for messaging functions; and so on.
An example of an API function in Windows NT is NetServerEnum. When a computer on a network issues a net view command to obtain the list of resources or computers that can be accessed using Network Neighborhood or Windows Explorer, the client computer issues a NetServerEnum API call to the Computer Browser service.