DocBook is a DTD - Document Type Definition. now what is this? well 
                   say for example  having said XML is in itself a rule set , suppose i use an 
                   markup element tag <author> in my document and another author uses <writer> element 
                   tag in his document, isn't it true we are trying to convey the same meaning .Imagine thousands of 
                   pages are being written for the web, for the publishing industry  and what a waste of enormous 
                   time if people would like to convey similar meaning but use different elements with the core 
                   language being same, this is where an DTD comes into picture. 
                 
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                       Docbook DTD is a very popular set of tags for describing books, 
                       articles and other prose documents, particularly technical documentation. Docbook 
                       is defined using the native DTD syntax of SGML 
                       and XML. Like HTML, DocBook is an example of 
                       a markup language defined in SGML/XML.
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| ---From the book  DocBook -
                     The Definitive Guide   by Norman Walsh and Leonard Muellner. |  |