TeX is a typesetting tool. Based on a text including a special markup (not SGML style) it produces printable output. TeX includes an entire programming environment within it.

The most popular extension is probably LaTeX. Emacs supports TeX, LaTeX and Texinfo files with their own major modes, ‘tex-mode’, ‘latex-mode’ and ‘texinfo-mode’. Actually, latex-mode is very similar to tex-mode. See also AUCTeX, a popular (La)TeX editing mode / environment for emacs.

Another extension to TeX is SliTex? which also has its own ‘slitex-mode’.

TeXmacs is a more conventional WYSIWYG word processor that is inspired by Emacs with regard to its keybindings, and by LaTeX with regard to math support. It uses TeX fonts and typesetting algorithms for its operation, but is neither actually using either Emacs or TeX/LaTeX. It has its own document format, from which it can export and import with varying success to LaTeX and HTML.

CategoryModes CategoryTex