From the official GNU Project Site:
GNU Emacs is an extensible, customizable text editor—and more. At its core is an interpreter for Emacs Lisp, a dialect of the Lisp programming language with extensions to support text editing.
It provides features common to programming text editors, including syntax highlighting, checking, language specific modes, and revision control.
Emacs makes use of a large set of keyboard shortcuts, such as Ctrl+F for moving forward one letter and Alt+B for moving backwards one word.
Other features that are used by programmers include:
- Shells running inside emacs
- Directory browsing
Some read email, news, twitter, IM or even web sites within Emacs.
Emacs is one of the two editors involved in the editor wars, along with vi.