Video: Matt Cutts Explains "Canonical Tag" from Google. Google, Yahoo & Microsoft Unite On “Canonical Tag” To Reduce. The web is full of duplicate content.
Search engines try to index and display the original or “canonical” version. Searchers only want to see one version in results. And site owners worry that if search engines find multiple versions of a page, their link credit will be diluted and they’ll lose ranking. Today, Google, Yahoo and Microsoft (links are to their separate announcements) have united to offer a way to reduce duplicate content clutter and make things easier for everyone.
Webmasters rejoice! PostRank. 360i Search Engine Marketing - Natural and Paid Search Optimizat.