This powerful typesetting system was created by Donald Knuth of Stanford University. Authors and publishers worldwide use TeX to produce high-quality technical books and papers. TeX does a superior job of formatting complex mathematical expressions.
The power of TeX lies in its ability to handle complicated technical text and displayed mathematical formulas. When coupled with a high-quality phototypesetter, TeX produces results equal in quality and appearance to those produced by the finest traditional typesetting systems.
TeX input files are ASCII-coded representations. They are easily transported, thereby facilitating manuscript sharing among authors, editors, reviewers, and publishers. This capability enables research results to be published and distributed electronically.
More details, including some history, can be found in Wikipedia, the free on-line encyclopedia.
An excerpt from The Advanced TeXbook, by David Salomon, gives a more technical overview.
Implementations of TeX exist for a wide variety of computing environments; this includes almost every current platform, and most older platforms still in use, often with multiple choices available for the most popular equipment. Implementations range from freeware and shareware (support may be offered via a dedicated mailing list) to commercial offerings (comprehensive, ready-to-run packages that include installation support). Nonetheless, the general rule applies that, regardless of implementation, the same input and processing setup (fonts, hyphenation patterns and the like) will yield the same output, subject only to the resolution of the output device used.
The AMS offers a collection of mathematical symbol fonts and two collections of style files -- AMS-LaTeX and AMS-TeX -- for use with TeX. These style files provide easy ways to code mathematical expressions. All these items are available via a Web link from the AMS TeX web pages or by anonymous FTP.