These include The HWND is the basic unit of visual display and input in the Windows developer platform.
Many functions in Windows USER manipulate properties on HWNDs such as size, position and title.
Each HWND also has a communication channel (WNDPROC - Window Procedure) for delivery of messages.
Developer functionality related to Windows USER is provided in the C header file winuser.h.
The file extension here was a misnomer, as Windows USER was in fact always a dynamic link library.