XEmacs was a branch version of Emacs that is no longer actively developed. XEmacs was first called Lucid Emacs, and was initially derived from a prerelease version of Emacs 19. In this FAQ, we use the name “Emacs” only for the official version.
XEmacs last released a new version on January 30, 2009, and it lacks many important features that exists in Emacs. In the past, it was not uncommon for Emacs packages to include code for compatibility with XEmacs. Nowadays, although some packages still maintain such compatibility code, several of the more popular built-in and third party packages have either stopped supporting XEmacs or were developed exclusively for Emacs.
Some XEmacs code has been contributed to Emacs, and we would like to use other parts, but the earlier XEmacs maintainers did not always keep track of the authors of contributed code, which makes it impossible for the FSF to get copyright papers signed for that code. (The FSF requires these papers for all the code included in the Emacs release, aside from generic C support packages that retain their separate identity and are not integrated into the code of Emacs proper.)
If you want to talk about these two versions and distinguish them, please call them “Emacs” and “XEmacs.” To contrast “XEmacs” with “GNU Emacs” would be misleading, since XEmacs too has its origin in the work of the GNU Project. Terms such as “Emacsen” and “(X)Emacs” are not wrong, but they are not very clear, so it is better to write “Emacs and XEmacs.”