DocBook is an XML format for writing documents. It allows the author to focus on the content itself during the writing process instead of worrying about the presentation.

Using standard DocBook tags, we can tag the content according to its syntatic structure. The DocBook document is then processed using XSL stylesheets so that each tagged DocBook element is transformed to a corresponding element in the target output format. For example each <para></para> element in DocBook could be transformed into a <p></p> element in XHTML.

Using different XSL stylesheets, we can generate different output formats. For example, we can generate both XHTML and PDF outputs from a single DocBook source. We can also generate multiple versions of XHTML (or PDF) files each with a different style if necessary.

XHTML is used instead of HTML as it ensures that the content is completely separated from its style using Cascading Style Sheets (CSS) and image files.