TeX is a computer typesetting system invented by Professor Donald E Knuth in the late 1970s. Despite its age, it remains at the forefront of computer typesetting technology, although by far the majority of its users elect to use LaTeX rather than TeX itself. Other so-called "formats" exist (LaTeX is a format) such as ConTeXt, AMSTeX, LamsTeX, Lollipop and so on, although none come near LaTeX in terms of popularity; ConTeXt, however, not only rivals LaTeX in terms of functionality, it may well exceed it.
TeX as defined by Prof. Knuth takes human-readable input, characterised by a proliferation of backslashes and braces, and produces as output a DVI file, where "DVI" is defined as "DeVice Independent". More recent systems, derived from and very closely based on TeX, include PdfTeX and XeTeX -- both of these take the same human-readable input as TeX but emit Adobe PDF rather than DVI (in the case of XeTeX, there is an intermediate XDVI (="eXtended DVI") representation which is then converted to PDF in a manner that is normally transparent to the end user). A further branch of the basic TeX code is represented by LuaTeX, which adds an interface to the Lua language accessible directly from the LuaTeX source.
This page is very much a work-in-progress; it will be enhanced whenever the present author and/or a fellow contributor has/have time.