Wikipedia
DocBook is a semantic markup language for technical documentation. It was originally intended for writing technical documents related to computer hardware and software but it can be used for any other sort of documentation.
As a semantic language, DocBook enables its users to create document content in a presentation-neutral form that captures the logical structure of the content; that content can then be published in a variety of formats, including HTML, XHTML, EPUB, PDF, man pages, Web help and HTML Help, without requiring users to make any changes to the source. In other words, when a document is written in DocBook format it becomes easily portable into other formats. It solves the problem of reformatting by writing it once using XML tags.