Note that there is a distinction--however subtle--between "window manager" and "desktop environment".
I have not seen a precise definition, so this is only mine:
Window manager=something that interfaces with the X server and allows GUI programs to open windows
Desktop environment=Window manager + tools to help organize things--eg utilties, file browser, panels, toolbars, etc.
In my diddling, KDE, Gnome, and XFCE are the big three Desktop Environments. Note that Xubuntu is Ubuntu + XFCE. (Is this the only example of a major distro that pacages something besides KDE and Gnome?)