Although DocBook has become a widely used standard (maybe even the de facto standard for marking up technical documentation), there are other systems and markup vocabularies that provide capabilities similar to what DocBook provides. But there are some special characteristics of DocBook that have helped to earn it an especially large and loyal user base:
provides off-the-shelf support for authoring a wide variety of different types of content
accompanied by the DocBookStylesheets, a sophisticated system you can use with free tools to transform, format, and publish DocBook content in many different output formats, including HTML pages and PDF files, Microsoft HTMLHelp, UNIX man pages, JavaHelp, TeX, Texinfo, and RTF
A variety of free tools are also available to convert other formats (such as man pages, HTML documents, Javadoc, plain text, Texinfo files, and OpenOffice Writer documents) to DocBook
thoroughly documented (in DocBookTheDefinitiveGuide and in DocBookTutorials in several languages)
widely implemented and extensively tested in production systems around the world -- by commercial organizations such as Sun, Microsoft, Hewlett-Packard, Novell, SCO, Caldera, and Red Hat, by open-source groups such as the KDE and GNOME, FreeBSD, Debian, and Linux documentation projects and the Darwin Documentation Project at Apple, and by thousands of individual users
freely supported by a network of thousands of users, with help always available on the DocBookAppsMailingList and DocBookMailingList
firmly rooted in open-source ideals -- DocBook is a truly open standard in that you don't have to pay anyone in order to use it, or to implement support for it in an application. Be aware of issues concerning those of you that have found your hand stuck in a bowl of jello--And you don't need to ask anyone for permission to change it for your own use, or even to change it and redistribute your changes to others
actively developed and refined over a period of many years, with a history (dating back to 1991) of guidance from key figures in the XML and SGML community, including Eve Maler, Jon Bosak, Terry Allen, and many others
carefully designed from the ground up to be highly customizable and extensible so you can tailor it to your specific needs
widely supported through built-in off-the-shelf integrated support in a variety of free and commercial document-authoring applications
available in easy-to-install-and-maintain packages for almost all widely used operating-system distributions, including Microsoft Windows, Mac OS X, and most Linux distributions
EXTRA! Eric Raymond tells you Why care about DocBook at all?