A window manager manages the actual windows on the desktop. The X Window System doesn't manage the windows on its own, it leaves that job to this program. Any graphical program you run makes a window. The window manager handles where a window will appear on the screen, how big the window will be, etc. It handles things like resizing and minimizing. The title bar at the top of most windows, and other window decorations, is often handled by the window manager.
In more recent times, the term composite window manager has appeared. This is a window manager that uses some new X extensions (most notable the CompositeExt) to alter the visual appearance of windows on the screen. A composite window manager can add real shadows to windows, make them see-thru, and perform other visual effects depending upon the window manager.
- Kwin (default window manager for KDE)
- Metacity (default window manager for some versions of GNOME version 2)