Description:

XEmacs (and regular GNU Emacs, too) is a self-documenting, customizable,
extensible, real-time display editor. XEmacs is self-documenting
because at any time you can type in control-h to find out what your
options are or to find out what a command does. XEmacs is customizable
because you can change the definitions of XEmacs commands to anything
you want. XEmacs is extensible because you can write entirely new
commands--programs in the Lisp language to be run by Emacs' own Lisp
interpreter. XEmacs includes a real-time display, which means that the
text being edited is visible on the screen and is updated very
frequently (usually after every character or pair of characters) as you
type.