Actually, I am not sure if it still exists, but CDE used to stand for Common Desktop Environment. IT was supposed to be a standard X client that would run on any unix variant. It would make it possible to sit down at any *nix, and be able to know where everything was.

There are many different clients out there. Sawfish, enlightenment, gnome, kde, etc etc. Gnome and KDE are just the most popular, and probably the best developed.