DocBook provides a system for writing structured documents using
SGML or XML. It is particularly well-suited to books and papers
about computer hardware and software, though it is by no means
limited to them.

In short, DocBook is an easy-to-understand and widely used DTD.
Dozens of organizations use DocBook for millions of pages of
documentation, in various print and online formats, worldwide.

Homepage:
http://www.oasis-open.org/docbook/
