.texi
or
.texinfo
extension.
Texinfo is a macro language, somewhat similar to LaTeX, but with
slightly less expressive power. Its appearance is of course rather
similar to any TeX-based macro language, except that its macros
start with @
rather than the \
that’s more commonly used in
TeX systems.
You can write and format Texinfo files into Info files within GNU
emacs, and read them using the emacs Info
reader. You can also format Texinfo files into Info files using
makeinfo and read them using info, so you’re not
dependent on emacs. The distribution includes a
Perl script, texi2html, that will convert
Texinfo sources into HTML: the language is a far better fit to
HTML than is LaTeX, so that the breast-beating agonies of
converting LaTeX to HTML are largely
avoided.
Finally, of course, you can also print the files, or convert them to
PDF using PDFTeX.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=texinfo