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. You can read
the on-line information, known as an "Info file", with an Info
documentation-reading program. By convention, Texinfo source file
names end with a .texi or .texinfo extension. 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.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=texinfo