Have you noticed those search results from Google that end up having images, description, and even rating with them? Well, that is Google Rich Snippets declared with microdata - a good way to specify to your search engine what the item is, and what are its properties.
The description of how microdata is to be used, can be found on Google microdata help page. But the basic idea is that relevant information is wrapped inside certain semantic tags, like this:
<div itemscope itemtype="http://data-vocabulary.org/Person">
My name is <span itemprop="name">Bob Smith</span>
but people call me <span itemprop="nickname">Smithy</span>.
Here is my home page:
<a href="http://www.example.com" itemprop="url">www.example.com</a>
I live in Albuquerque, NM and work as an <span itemprop="title">engineer</span>
at <span itemprop="affiliation">ACME Corp</span>.
One drawback though is that there is a problem with Drupal's markup as you edit your node.tpl.php to apply it. Sometimes, the analyzer fails to get the microdata markup from the series of wrapping spans and divs. The best solution is to simplify your node. There is an alternative solution - move the microdata in a hidden div. Though, this second approach is not very "clean" and optimizing your node markup is always more preferable.