Learn more at http://www.XEmacs.org
I think of the "X" in XEmacs as standing for Experimental. It's an independently maintained code fork that contains Emacs Lisp modules and other features that are still being tested before they are reimplemented in the official Emacs by somebody who's not too lazy to assign copyright to the Free Software Foundation (per standard policy for patches to GNU software).
At this rate, GNU Emacs is becoming a mere clone of XEmacs.
(thanks to
bis for the correction)