The basic major mode for (La)TeX included in GNU Emacs. Not to be confused with AUCTeX, a common third-party package.
TeX mode is a family of major modes to edit files for the TeX document processing system, including latex-mode for LaTeX files. It is included in GNU Emacs.
Use this tag for questions about the TeX Mode family (plain-tex-mode, latex-mode, slitex-mode, doctex-mode). For questions about TeX documents in general, see tex. For questions about LaTeX documents in general, see latex.
An alternative to TeX mode is AUCTeX auctex, a third-party package (included in XEmacs) with more advanced features including syntax highlighting and completion of macros in third-party packages, better support for complex compilations, preview of mathematics, etc.