You had the same question I did, only you asked it before I did!
After seeing Bill's answer, I Googled: Gnome vs KDE it resulted in a lot of good articles !!
Hope that helps! (I sent this earlier, but my message seems to have gotten lost)
They're very different desktop systems with some similarities and some similar goals. Honestly, the best thing you could do would be to download a couple different Linux distros to test each of them out and figure out the differences yourself.