What’s “writing in TeX”?
TeX is a macro processor, and offers its users a powerful
programming capability. To produce a document, you write macros and
text interleaved with each other. The macros define an environment in
which the text is to be typeset.
However, the basic TeX engine is pretty
basic, and is a pretty difficult beast to deal with. Recognising this
(and not wanting to write the same things at the start of every
document, himself) Knuth provided a package of macros for use with
TeX, called Plain TeX; Plain TeX is a useful minimum set of
macros that can be used with TeX, together with some demonstration
versions of higher-level commands. When people say they’re “writing
(or 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=plaintex