The top 3 search engines joined together to address a duplicate content issue many site owners may not even be aware they have. By utilizing the link tag to canonicalize domain names these search engines will clean up their indexes and display better search results.
Websites will also benefit in the search results because this will be treated like a 301 redirect.
This tag defines the preferred URL and tells search engines which canonical URL form to use when indexing site pages for search results. To implement the tag on your site just add the following link tag in the header section:
<link rel=”canonical” href=”http://www.example.com/products” />
By adding this link tag in the header Google, Yahoo, and MSN will disregard other URL variations on your site that have the same content as the preferred URL.
For further details and examples you can read more about this at the Google Web Master Blog.