Software documentation system
Texinfo is a documentation system that uses a single source file to produce both online information and printed output. It is primarily designed for writing software manuals.
For a full description of the Texinfo language and associated tools, please see the Texinfo manual (written in Texinfo itself). Most likely, running this command from your shell:
or this key sequence from inside Emacs: M-x info RET m texinfo RET will get you there.
or any GNU mirror site.