OS X is a tru OS, not a desktop Manager
Aqua is the Desktop Manager that Runs on OS X
exampleBSD, FreeBSD, AIX, Linux, Solaris, OS X are all Unix Based Operating Systems
KDE, GNOME, XWindows, Enlightenment and Aqua are all desktop managers, the gui that interacts with the os so you don't do everything in command line.
You can compile X windows to run on OS X which means in theory you can also compile KDE and Gnome (not that you'd want to, to me they remind me of winblows)
Now take into consideration I may be wrong, but thats how I understand it.