As you have seen above, DocBook is similar in file structure to HTML. Although if you are used to HTML, DocBook will probably seem pretty tag-verbose to you. DocBook also uses logical tags, it has no (or at least very few) layout-specific tags like HTML is full of. The idea with DocBook is to tell as much as you can about the information while writing it, so that software can do more things with it. In DocBook you use tags to mark the meaning of parts not the presentation.
We use DocBook, because it eases the generation of the several formats of the documentation, and acts as a central "meta format" for all the things we would like to express in the documentation.
You cannot just invent tags in DocBook. There is a standard set of tags you can use. If you are working on a translation, please do not translate tags, as they need to stay as they were written in the English files to work properly.