Window Manager is the part of the Windows NT Executive that creates the familiar screen interface. It is also responsible for processes, like messaging, that use windows functions without ever affecting the user interface.
The Window Manager (USER) and Graphics Device Interface (GDI) functions are implemented by a single component in the Windows NT Executive: Win32k.sys, known as Winsrv.dll in previous releases of Windows NT.
Applications call the standard USER functions to create windows and buttons on the display. Window Manager communicates these requests to GDI, which passes them to the graphics display drivers where they are formatted for the display device.
The Window Manager notifies applications when changes occur in the user's interaction with the interface, such as movement or resizing of windows, cursor movement, and icon selection.
Prior to Windows NT 4.0, the Window Manager was the USER component of the Win32 subsystem and ran in a protected process in user mode. With this release, they were moved into the Windows NT Executive to run in kernel mode. This change was designed to speed up graphics calls and reduce the memory requirements of interprocess communication.