First is the Structure, which is what is commonly called the DTD, or Document Type Definition. The DocBook DTD, used to create this document, is an example of this. The DTD lists the rules that the content must follow. Finally there is the Content, which is what gets rendered by the SGML processor and is eventually seen by the user. This paragraph is content, but so would a graphic image, table, numbered list, and so on.
Content is surrounded by tags to separate out each element.