Google recently announced support for a new meta tag to designate canonical URLs as an alternative to using 301 Permanent Redirects. For those unaware, a canonical URL is the primary URL you want your content indexed as, often times duplicate content can be found on multiple URLs, especially when using a CMS or eCommerce platform. If a product exists in multiple categories, there may be multiple URLs for that product, all with virtually identical data. In certain situations, such as the product in multiple categories, a redirect to a canonical version may not be ideal, and in other situations, it may not be possible to set up appropriate redirects. For these circumstances, the new meta tag was created. The format for the new tag is as follows:
<link rel="canonical" href="http://www.mystore.com/my-book/" />
You would place this in the header of all similar pages. As an example, http://www.mystore.com/books/my-book/ and http://www.mystore.com/authors/andrew/my-book/ should both include the meta tag. This should also help with the myriad of tracking variables that can show up on the end of URLs.
You can use the meta tag across subdomains, thus http://books.mystore.com/my-book/ can point back to http://www.mystore.com/my-book/, but you cannot use it across domains, so http://www.mybooks.com/my-book/ will not work.
Relative URLs are allowed, however I would strongly recommend specifying an absolute URL, the entire point of the meta tag is to specify a URL that is the definitive canonical version of a page, best to specify it absolutely to reduce the chance of errors.
The meta tag is supported by all the major search engines: Google, Microsoft Live, Yahoo!, and Ask.
For more details, see Google’s excellent writeup on specifying your canonical.