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 including details about the win32 module structure.