DocBook is an XML/SGML vocabulary particularly well suited to books and papers about computer hardware and software (though it is by no means limited to these applications).
The DocBookTechnicalCommittee maintains the DocBook schema.
DocBook is officially available as a Document Type Definition (DTD) for both XML and SGML. It is unofficially available in other forms as well.
You can download both the latest DocBook XML DTD and DocBook SGML DTD from the official DocBook site at OASIS.
DocBook enables you to author and store document content in a presentation-neutral form that captures the logical structure of the content. Using free tools along with the DocBookStylesheets from the DocBookProject and related resources, you can transform, format, and publish your content as HTML pages and PDF files, and in many other DocBookOutputFormats, including Microsoft HTMLHelp, UNIX man pages, JavaHelp, TeX, TeXinfo, and RTF. (Free tools are also available to do "up conversion" of other formats such as man pages, HTML, Javadoc, plain text, Texinfo, and OpenOffice Writer documents to DocBook.)