When to use
Emphasis tags (<strong> and <em>) should be used to indicate additional importance or emphasis of a word or phrase relative to its surrounding copy. Use <strong> for importance and <em> for stress emphasis.
Emphasis in a paragraph
Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Maecenas faucibus mollis interdum. Nulla vitae elit libero, a pharetra augue.
Note: It's still okay to use <b> and <i> tags in HTML5 and they don't have to be styled bold and italic, respectively (although if there is a more semantic element, use it). <b> is meant to highlight words or phrases without conveying additional importance, while <i> is mostly for voice, technical terms, etc.
Addresses
The <address> element is used for contact information for its nearest ancestor, or the entire body of work. Here are two examples of how it could be used:
University of Lincoln
Brayford Pool
Lincoln
LN6 7TS
Tel: (+44) 01522 882000
Note: Each line in an <address> must end with a line-break (<br />) or be wrapped in a block-level tag (e.g., <p>) to properly structure the content.
Abbreviations
For abbreviations and acronyms, use the <abbr> tag (<acronym> is deprecated in HTML5). Put the shorthand form within the tag and set a title for the complete name.