Texinfo is a documentation system that uses one source file to produce
both on-line information and printed output. So instead of writing
two different documents, one for the on-line help and the other for a
typeset manual, you need write only one document source file. When
the work is revised, you need only revise one document. By
convention, Texinfo source file names end with a 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