Don't forget, you can press \ to view the HTML source.
A line break is used when you want to start a new line, but don't want the extra white space that is used to separate paragraphs. Here is an example:
Some Company Inc.
1234 Anywhere Street
Italics and boldface can be used to emphasize text. In theory, the tag for emphasized text can be used. This is considered more "correct" (because it describes the function rather than the appearance), but in practice, not all browsers recognize it.
Finally, the horizontal rule is useful for drawing horizontal lines to separate the parts of your document. Here is an example: