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 provide a 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 inegrated with a Desktop Environment.
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:
Other window managers: