What is the difference between GNOME, KDE, Xfce, and LXDE? - Renewable PCs

What is the difference between GNOME, KDE, Xfce, and LXDE? - Renewable PCs:

Desktop Environment / Window Manager	RAM used	% of CPU (2.6 GHz total) used	Type
KDE 4.6	363 MB	4 %	desktop environment
* Unity (Ubuntu 11.04)	271 MB	14 %	desktop environment
* GNOME 3.x shell	193 MB	10 %	desktop environment
GNOME 2.x	191 MB	1 %	desktop environment
Xfce 4.8	144 MB	10 %	desktop environment
IceWM	85 MB	2 %	window manager
LXDE	85 MB	1 %	desktop environment
Enlightenment (E17 Standard)	72 MB	1 %	window manager / desktop shell
Fluxbox	69 MB	1 %	window manager
OpenBox	60 MB	1 %	window manager
JWM	58 MB	1 %	window manager

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