GNU Emacs (short for Editing Macros) is a text editor that is common on many UNIX-based operating systems.
Emacs is primarily used by programmers.
Emacs is made powerful by Emacs Lisp, a built-in programming language that lets the user extend the capabilities of the editor.
A common Emacs joke is that all of the functions of the editor are crazy weird keystrokes (such as "control-meta-4 shift-left-P-semicolon-F1" to do something simple like cut and paste text). In reality, though, these keystrokes are relatively simple, though they can take some getting used to.
There is an Internet turf war between programmers that prefer Emacs and programmers that prefer Vim (or Vi), another common text editor.