A window manager is a program that controls the placement and appearance of application windows under the X Window System.
Modern computer systems generally offer a graphical user interface that enables a user to interact with a number of application programs simultaneously; each one typically has its own independent window.
The Microsoft Windows and Apple Macintosh platforms have historically provided a vendor controlled, fixed set of ways to control how the windows are displayed on the screen, and how the user may interact with them. However, the X Window System, popular under Unix systems, allows the user to choose between various window managers to achieve different behavior from the program windows. Window managers differ from one another in several ways, including appearance (including menus, docks, pagers, etc.), memory consumption, customizability, multiple and virtual desktop (desktop larger than the physical monitor size) support and whether they are closely integrated with a Desktop Environment.
A variety of window managers are currently available for X, ranging from the bare-bones TWM (which was the very first window manager) to the rather elaborate Enlightenment.
There have been efforts to produce alternative shells for Microsoft Windows. For example, LiteStep can replace the UI on Windows 95, 98, or NT with an Afterstep style. OS/2 ships with Presentation Manager as the default shell, but others are available from third party sources.
Popular window managers:
- Kwin (originally called Kwm, default window manager for KDE)
- Metacity (default window manager for GNOME) 
- Sawfish (originally called Sawmill)