Emacs is a fairly ubiquitous editing environment for Linux. It's been ported to all modern operating systems ( Linux, Windows, OSX, even DOS ). It's more than a text editor, as it has its own built-in tools: a file manager, organizer, calculator, journal, e-mail client, IRC client, tetris game, and lots of other stuff. In order to have this extensibility, it has its own built-in programming language: Emacs Lisp.
Since Elisp lives inside Emacs, it's a great tool for quick and dirty front-ends for back-end tools or web APIs. Since it's feature-rich, it's good for coding toys and games. Lastly, since it's Lisp, it's suitable for academic computing and artificial intelligence.