Duplicate content has been a challenge for a long time for website owners and search engines. Often blogs end up with several urls for each post making it difficult for search engines to crawl only one unique content. So thanks go to the joint effort from Google, Yahoo and MSN to create this new Canonical URL Tag.
Simply add the tag in the HTML header on a web page, just below the Meta Description tag
<link rel="canonical" href="http://www.pimpmywordpress/page.html"/>
This tag will tell search engines that the preferred location of this url is http://www.pimpmywordpress/page.html instead of http://www.pimpmywordpress.com/page.html?sid=123.
If you are still maybe a bit confused check this video out by Matt Cutts of Google.