DocBook is a schema (available in several languages including RELAX NG, W3C XML Schemas, and XML DTDs) maintained by the DocBook Technical Committee of OASIS. It is particularly well suited to books and papers about computer hardware and software (though it is by no means limited to these applications).
Historically, DocBook was a DTD. Some older versions are only available in that format.
The V5.x schemas are the focus of current development work.
DocBook V5.0 is an OASIS Standard.
DocBook V4.5 is the last of the DTD-based versions of DocBook. It is described normatively with XML and SGML DTDs. (There are no official XML versions of DocBook prior to V4.1.2.)
DocBook V4.5 is also an OASIS Standard.
Each release of DocBook includes a specification. The list of current (and historical) specifications is available online.