GNU Emacs is a free, portable, extensible text editor. That it is free means specifically that the source code is freely copyable and redistributable. That it is portable means that it runs on many machines under many different operating systems, so that you can probably count on being able to use the same editor no matter what machine you are using. That it is extensible means that you can not only customize all aspects of its usage (from key bindings through fonts, colors, windows, mouse, and menus), but you can program Emacs to do entirely new things that its designers never thought of. Because of all this, Emacs is an extremely successful program, and does more for you than any other editor. It is particularly good for programmers. If you use a common programming language, Emacs probably provides a mode that makes it especially easy to edit code in that language, providing context sensitive indentation and layout.