Win32 is the 32bit Windows/ReactOS subsystem responsible for executing x86 executables.
The Core of the Win32 subsystem is found in the following modules
- kernel32.dll (Base services)
- gdi32.dll (Graphics Device Interface)
- user32.dll (User interface)
- win32k.sys (Kernel level support for the graphics device and user interfaces)
There are roughly 40 other libraries that make up the rest of Win32.
Wikipedia has a nice article with focus on the Windows API includong details about the win32 module structure.
See Arwinss page for the alternative win32 subsystem implementation effort.