web
source into something
that can be compiled and something that can be printed; there’s never
any mystery about what TeX does. Furthermore, the WEB system
provides mechanisms to port TeX to new operating systems and
computers; and in order that one may have some confidence in the ports,
Knuth supplied a test by
means of which one may judge the fidelity of a TeX system. TeX
and its documents are therefore highly portable.
TeX is a macro processor, and offers its users a powerful
programming capability. For this reason, TeX on its own is a
pretty difficult beast to deal with, so Knuth provided a package of
macros for use with TeX called Plain TeX; Plain TeX is
effectively the minimum set of macros one can usefully employ with
TeX, together with some demonstration versions of higher-level
commands (the latter are better regarded as models than used as-is).
When people say they’re “programming in TeX”, they usually mean
they’re programming in Plain TeX.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=whatTeX