The window manager is the piece of software that controls the windows in the X window environment. The placement, borders, and decorations of any window are managed by the window manager. This is very different from many other operating systems, and the way GNOME deals with window managers is different from other desktop environments.
As stated earlier in this guide, GNOME is not dependent on any one window manager. This means that major parts of your desktop environment will not change when you decide to switch window managers. GNOME works with the window manager to give you the easiest work environment you can have. GNOME does not worry about window placement but gets information from the window manager about their placement. The GNOME Pager will only work with a GNOME compliant window manager as will drag and drop on the desktop.
At the time of this version of the GNOME User's Guide the Enlightenment Window Manager is the only window manager that is 100% compliant. There are many other window managers that are partially compliant or are being modified to meet compliance.
Some of the window managers that have partial to full compliance at the time of this version of the GNOME User's Guide are:
Enlightenment - http://www.enlightenment.org
Window Maker - http://www.windowmaker.org
FVWM - http://www.fvwm.org/
AfterStep - http://www.afterstep.org/
There are a host of newer window managers being developed that will work with GNOME. You can find a list of these on the GNOME Software Map.