Emacs is the extensible, customizable, self-documenting real-time display editor. – the EmacsManual

What does this mean? Extensible refers to Emacs being built upon a core set of routines hard-coded in C, which are then extended using a language called EmacsLisp. For this reason, Emacs remains the most powerful editor ever made - you can customize it to suit your needs perfectly. If there isn’t a feature, just add it! Emacs can do anything that can be coded in EmacsLisp, and that is a heck of a lot of things.

Here is a short list of Emacs features.

UndoHypertext Archive look-up
Copy/Paste Alignment Calendar, diary
Mark ring Sort text Math computation
Syntactic moves Numbering Emacs scripting
Search and replace Spelling Shell interface
Regexp handling Templates Session management
CompletionOutline Planner
Keyboard macrosAbbreviations Read mail and news
Highlight text Tables Browse man and info
Parenthesis match File management Online documentation
Section formatting Speedbar Print
Rectangular selection Programming modes Chat
Indentation Debug IRC client
Registers, bookmarks Version control Draw ascii
TagsFTP interaction Play Tetris

More reading on useful Emacs features:

Why choose Emacs over other editors:

External links