winows.h is a Windows-specific header file for the C programming language which contains all the function declarations in the API, as well as declarations for all the common macros used by windows programmers, and all the data types used by the various functions and subsystems. It defines a very large amount of windows specific functions that can be used with C. The Win32 API can be added to a C programming project by linking to the appropriate libraries (kernel32.lib, user32.lib and gdi32.lib), and by including the <windows.h> header file.