One of the most significant changes in HTML 5 is a cleaner system of identifying different elements and their purpose on a web page. As I have learned HTML and CSS, I have always had a very small thought that maybe "divs" needed a more clearer purpose since they are pretty much being used for everything. Well HTML 5 takes care of this problem by implementing the logical "header", "footer", "nav", "article", "section", "audio", "video" and "aside" tags that clearly identify the named element AND its purpose.
This may mean little to a programmer, but to a search engine, it is just one more clue into the overall identity of a webpage and its purpose. Content will be as easy to parse as an “img” tag with a good “alt” attribute. Although there is still little evidence that these new of semantic tags will directly increase search engine rankings, it will, logically, produce better quality search results.
Along with HTML 5, WC3 has more in store for SEO. It also promises a new form of indexable data called “microdata”. Microdata is a more specific way to tell a search engine what the content on a website means. For example, you can explicitly tell a search engine that the numbers on your website are prices and not dates. You can even add information about the specific contents of a recipe, product reviews, or events to the content on your page. The defined microdata types can be found on schema.org.
Schema.org is a new, on-going, joint project in between Yahoo, Google, and Bing to define types of microdata for web use. Currently, only a small hand full of data-types is in use at the moment, but they are being slowly implemented with HTML 5. There is even cross-platform functionality, such as the ability of an “event” microdata to be seamlessly added to iCal or Google calendars. I believe it is my prerogative to start implementing and learning microdata. And if not microdata then surely you should be familiar with RDF's, and microformats...No? Well here's a handy source that explains them, if you're into the techy stuff: Rich Snippets