DocBook defines a set of markup elements useful for marking up text so that the text can then be transformed into several different formats: HTML, XML, RTF, TeX, and others.
The idea is to write just once and reach the largest possible number of people with the information.
Digital information not stored properly tends to get lost. Because DocBook uses standard ASCII characters, it is easy to index and search DocBook document directly.
The SGML systems use markups to describe their data. DocBook holds over 300 markup elements each one with several attributes which can assume several values; these can be fixed or defined by the document / style that the author has used.
Just to remind you, if any changes are made to DocBook's DTD, it's no longer DocBook.