While I'm not an expert, here is the most I know about your question:
The X Window System (aka, X Window, X or the ever controversial X Windows), is a method of enabling graphic display on a Unix-like system. It is set up in a client/server model (although the reverse of what one would expect; the client is the one dishing out graphics, and the X Window server is the machine "serving," or displaying, the graphics to you) to allow graphics display on any machine on a network. XFree86 is an open source variant of the X Window System.
Since X only allows graphics to be displayed (it only has a rudimentary windowing system of its own) other bits must be added to create a "desktop environment." First, a window manager is usually added (Enlightenment, WindowMaker, et al). This program manages windows (imagine that
). It allows themeing of windows (they can look like anything), and works a bit with placement. Next a desktop environment can be added. This is where KDE and Gnome come into play.
Gnome and KDE provide a more integrated environment, combining the Graphics layer (X Window), window manager (Enlightenment or other), a program to browse the hard drive or network, and a host of other utilities for set up, clocks, Taskbar/Dock like (I know I'll be pummeled for saying this) and many of the other bits needed for other programs to run.
By compairison, Mac OS X has the Quartz layer controlling graphics and, to the best of my knowledge, windows and the Finder/Desktop and the Dock for other Desktop environment functions (this is a simplification).
The point of all this: to run one of the open source graphics apps which run under KDE or Gnome, one needs to have KDE or Gnome (or at least their libraries?) installed on top of X Window. Basically, you can't run both Quartz and X Window at the same time. However, X Window can run "underneath" OS X as a client, with an X Server optimized for Quartz/OS X. This is what Tenon provides with X tools. The other option is Xvnc. To find out more about it, check out this thread:
http://forums.macnn.com/cgi-bin/Forum3/HTML/001268.html
I hope that helps.
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