HTML5 Microdata: What is it, and why should you care? Good intro on microdata in HTML5 and how it’s going to let you mark-up documents so crawlers can determine context, not just content.
In short, microdata allows the user to create “custom” elements (sort of), to give a specific meaning to them.
Consider these standard properties to describe a person. You can embed these markers in HTML, so any search engine can understand them too.