This information is condensed from the release notice:
Lucid GNU Emacs is based on an early version of GNU Emacs version 19 with many enhancements. It currently requires X Windows to run. For information on where to get Lucid GNU Emacs see 121. X Windows support is greatly enhanced over GNU Emacs version 18, including support for multiple X Windows (a.k.a. screens in Emacs), Zmacs/Lispm style region highlighting, a customizable, Motif-like menubar, more powerful keymap support (allowing different actions to be associated with Backspace, Control-h, etc.), flexible text attribute (e.g. font, color) support on regional and screen-local basis through X resources and/or lisp, and support for the X11 selection mechanism. Some other features include run-time computation of the load-path, support for floating point numbers, native timer support, and sound file support on Sun SPARCstations. To build Lucid GNU Emacs, an ANSI C compiler (e.g. gcc) is required.