The bullets are those little dots in front of each entry. Look at the bottom of this page. The sizes 1 through 6 are relative. That is, Heading 1 is always bigger than Heading 6, but the actual sizes of these will vary depending on the computer used to look at the document. This lets you start a new line, but each line is still in the same paragraph.
Line breaks put less vertical space between lines than paragraph breaks.
Paragraph breaks let you change formatting. You can use tables to organize your material. In particular, you can use tables to put material into columns, lists or boxes without using frames. This lets you present clear pages that load quickly. The UT Homepage is a great example of the use of tables to organize content.