Google, Yahoo, and Microsoft announced a joint effort to help reduce duplicate content. The three major search engines came together to allow users to point out their preferred version of a url. This format offers users more control.
Duplicate content has been a challenging issue for a long time. Websites containing a lot of content such as a retail site, could end up with several urls for each page making it difficult for search engines to crawl.
Google gives the following example on the Webmaster Central blog:
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
Simply place this link tag in the head section of the duplicate content urls.
The tag can only be used on pages within a single site. Both absolute and relative links are acceptable, but the search engines recommend absolute links. Also, links to all urls will be directed to the one preferred url.