A WindowManager is a program that, well, manages your windows...
If you loaded up an X server without a WindowManager, then you wouldn't be able to move or resize windows. The WindowManager creates the border around the window, allowing you to resize it as well as minimize, maximise, or close the window.
Some WindowManagers can be run standalone as a lightweight X solution. These include:
Others are intended to be used with a DesktopEnvironment:
32 pages link to WindowManager:
