What Desktop do you prefer?
KDE is more like Windows while Gnome is more a BSD/Apple Type Desktop, Unity is the default supplied by Ubuntu which has taken a lot of flack and then you have things like X Windows and so on.
http://en.wikipedia.org/wiki/KDEhttp://en.wikipedia.org/wiki/GNOMEhttp://en.wikipedia.org/wiki/Unity_(user_interface)
http://en.wikipedia.org/wiki/XWindows