What is a window manager, and what does it have to do with GNOME?

A window manager is a program within the X Windows environment which controls the placement and appearance of windows on the screen. Look at an xterm or something. The box of terminal text is the actual xterm, the border, title bar, buttons and so forth are all handled by the window manager. Some window managers have extra features, but all handle window placement and decoration.

There are many window managers out there, and GNOME should work well with all of them. There are a few GNOME features (such as session management, and the GNOME pager), that won't work with all of them. For best results, go with a GNOME-compliant window manager. The two most compliant are currently Enlightenment and IceWM, with FVWM, SCWM, Window Maker, AfterStep and QVWM putting finishing touches on GNOME-compliance as we speak.