DocBook defines a set of markup elements useful for marking up text so that the text can then be transformed into several different formats.
It's possible to create documents in 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. Due to the fact that not containing uncommon characters (such as binary formats) it's possible to index and search directly on the documents written on SGML and consequently on DocBook.
The SGML systems use markups to make their description. 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 if any changes are made on the DocBook's definitions DTD, it's no longer DocBook.