The Windows 32-bit API set is a portable API set that exposes a flat 32-bit address space. The API set is initially on Windows NT, and over time will be available on MS-DOS.
Win32 makes a clear distinction between 32-bit applications and 16-bit applications. Mode mixing is not allowed (i.e. a 32-bit application may only use Win32 functions and may not make calls into 16-bit DLLs). This design encourages portability. The portability goals for Win32 are very simple: an application that calls the Win32 API set should only have to recompile in order to run on Windows NT for MIPS.