Emacs is a set of text editors controlled by Emacs Lisp. The two main types of Emacs are GNU Emacs and XEmacs. Usually, people just call them "Emacs".
Gnu Emacs calls itself an "extensible, customizable, self-documenting, real-time display editor."
Features of both forms of Emacs include syntax highlighting for a variety of programming languages, easy movement through documents, and keyboard macros.
Plugins and libraries can be used in conjunction with emacs to make it do more interesting things like diffs, play text games, and view email.