First is the Structure, which is what is commonly called the DTD, or Document Type Definition. The DTD defines the relationship between each of the elements. The LinuxDoc DTD, used to create this document, is an example of this. Second 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 different element.
Over time, the LinuxDoc DTD is going to change over to the DocBook DTD, used by others and giving the LDP a consistent look and feel to other SGML documentation. SGML provides for more than just formatting. You can automatically build indexes, table of contents, and links within the document or to outside. In the end, SGML is more concerned about the way elements work instead of the way they look. A big distinction, and one that will let you write faster, since you don't have to worry about placement of paragraphs, font sizes, font types, and so on.