The following is extracted from arXiv help

TeX, LaTeX, etc.

TeX is a text formatting and markup language developed by D. Knuth in the late 1970s and early 1980s. It has become the defacto standard for document preparation in mathematics and certain areas of the physical sciences because of its unsurpassed utility in typesetting mathematical formulae. TeX allows extension through user-provided or third-party-provided packages of macro definitions.

LaTeX is an extension of TeX, which for purposes of discussion can be thought of as a third-party macro package for TeX. The goal of LaTeX is to provide a logical system for document preparation, where the author does not have to give specific formatting instructions in order to achieve a particular look; the look is provided by an external document class description.

There are other variants of TeX, and many user-provided contributions. Tex and its variants have evolved into a large and complicated suite of document preparation tools.

A great deal of information is available on the Web. Check our detailed utility help for links to TeX on the Web.

TeX and LaTeX files usually have a .tex extension.