The Win32 API, or WinAPI, is classified as the entire Windows library and its functions, branching from windows.h, it includes GDI (wingdi.h), and many other core windows platforms. Most windows programs require Win32 at the core of operation, from simple windows to giant games. The WINAPI WinMain function is the Windows equivalent of int main(), and has its own syntax that all windows programs must follow. First, programs must include windows.h.