The .TEX file extension is used by LaTeX, a document preparation system for the TeX typesetting program.

It offers programmable desktop publishing features and extensive facilities for automating most aspects of typesetting and desktop publishing, including numbering and cross-referencing, tables and figures, page layout, bibliographies, and much more.

TeX's author is Donald Knuth a very famous computer scientist. Leslie Lamport wrote LaTeX, a set of TeX macros very easy to use. Today many people share their packages for specific tasks like music score (partitures), chemical formulas, comnuting diagrams in maths, etc. Thanks to this colaboration it is possible to obtain very profesional results with a minimal effort.

Read the not so short guide to LaTeX and start up in just one day.