Quote from the introduction part of DocBook: The Definitive Guide:
DocBook 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.
Simply this means, that writing a DocBook file is no more, than writing a text file, using some simple "tags" where they are needed. An opening tag is a < bracket followed by the tag's name, optinally some arguments, and a > bracket (eg. <para> for paragraphs). A closing tag is a < bracket followed by a / sign, the tag name and a closing > bracket (eg. </para> for paragraphs). The best way of studying what tags to use is to look into the manual files, or the collected samples in the Documentation Skeletons section of this HOWTO. You can also download a full copy of the DocBook book mentioned above, so you can search for appropriate tags to use yourself.
We use DocBook because it is so easy to generate several output formats of our documents (HTML, PDF, Microsoft HTML Help, RTF, etc.), and is a world standard way of storing structured technical information.
You don't need to know all tags of DocBook to contribute to our work. If you are a translator, you need not translate any tags or invent others, so you will only be interested in the content between the opening and closing tags.