dwm

Windows Desktop Manager runs as a service - dwm session manager (internal name UxSMS - User Experience SMS).  dwm compose the desktop.  In past version, including Windwos Vista not running Aero, each application write directly to the frame buffer.  When an application windows is overlaid by another application, Windows will send a WM_PAINT message to the affected application to ask it to redraw its window on screen.  If the application is busy and does not handle the WM_PAINT message, it may result in a trial of the moving window image and this situation is called "tearing".

With dwm, each application is allocated with its own frame buffer.  dwm took these images and produce a composite image to display. dwm is also responsible to give a shade of wait over an unresponsive windows, called ghosting.