GNU Emacs is the extensible, customizable, self-documenting real-time display editor. This description is from the "Introduction" section of the Emacs Manual.
We say that Emacs is a "display" editor because normally the text being edited is visible on the screen and is updated automatically as you type your commands.
We call it a "real-time" editor because the display is updated very frequently, usually after each character or pair of characters you type. This minimizes the amount of information you must keep in your head as you edit.
We call Emacs advanced because it provides facilities that go beyond simple insertion and deletion: filling of text; automatic indentation of programs; viewing two or more files at once; and dealing in terms of characters, words, lines, sentences, paragraphs, and pages, as well as expressions and comments in several different programming languages.
"Self-documenting" means that at any time you can type a special character, `Control-h', to find out what your options are. You can also use it to find out what any command does, or to find all the commands that pertain to a topic.
"Customizable" means that you can change the definitions of Emacs commands in little ways. For example, if you use a programming language in which comments start with `<**' and end with `**>', you can tell the Emacs comment manipulation commands to use those strings. Another sort of customization is rearrangement of the command set. For example, if you prefer the four basic cursor motion commands (up, down, left and right) on keys in a diamond pattern on the keyboard, you can have it.
"Extensible" means that you can go beyond simple customization and write entirely new commands, programs in the Lisp language to be run by Emacs's own Lisp interpreter. Emacs is an "on-line extensible" system, which means that it is divided into many functions that call each other, any of which can be redefined in the middle of an editing session. Any part of Emacs can be replaced without making a separate copy of all of Emacs. Most of the editing commands of Emacs are written in Lisp already; the few exceptions could have been written in Lisp but are written in C for efficiency. Although only a programmer can write an extension, anybody can use it afterward.
/usr/local/bin/emacsto get our version.
emacs [ -nw ] [ <filename> ]
To save a file, type Ctrl-X Ctrl-S. To exit emacs, hit Ctrl-X Ctrl-C.