Texinfo

Texinfo is a typesetting syntax used for generating documentation in both on-line and printed form (creating filetypes as dvi, html, pdf, etc., and its own hypertext format, info) with a single source file. It is implemented by a computer program released as free and open source software of the same name, created and made available by the GNU Project from the Free Software Foundation.

The main purpose of Texinfo is to provide a way to easily typeset software manuals. Similar to the LaTeX syntax, all the normal features of a book, such as chapters, sections, cross references, tables and indices are available for use in documents. Using the various output generators that are available for Texinfo, it is possible to keep several documentation types up-to-date (such as on-line documentation provided via a Web site, and printed documentation, as generated using the TeX typesetting system) using only a single source file. As the manual notes, "TeX works with virtually all printers; Info works with virtually all computer terminals; the HTML output works with virtually all Web browsers. Thus Texinfo can be used by almost any computer user."

Read more about Texinfo:  Output Formats, Texinfo Source File, History and Status