X Window Manager

A striking difference between X11 and other window systems is that X11 does not integrate control of X clients in the X server, but rather concentrates it in an independent X application, namely the X window manager. Like window managers of other window systems, it controls the assignment of input devices to an X client and helps the user in modifying various window attributes such as position and size, an X application converting to an icon or vice versa, and positioning a window in the window stack (raising or lowering it).