From ReactOS

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.