Windows API

API is an acronym for Application Programming Interface.  Consists of the functions, messages, data structures, data types, and statements you can use in creating applications that run under Microsoft Windows.  The parts of the API you use most are code elements included for calling API functions from Windows.  This includes procedure declarations (for the Windows functions), user-defined type definitions (for data structures passed to those functions), and constant declarations (for values passed to and returned from those functions).