Win32 is the common name for all 32-bit versions of Microsoft Windows. These include all Windows 9x versions (Windows 95, Windows 98 and Windows ME (ME is short for Millennium edition)) as well as all versions of Windows NT (that includes Windows 2000 and Windows XP).
Windows versions prior to Windows 95 that are not versions of NT use a 16 bit architecture and are therefore called Win16.
Win16 is (was) in reality more of a shell to DOS than a new operating system even if it is a shell that adds substantial amounts of functionality. In the same sense is X a graphical shell to Linux, this is not true for Win32 even if the Win9x series have large sections of code that are similar to DOS and a startup routine that suggests that DOS is still a large part of it.