The newly released Canonical URL Tags, supported by all three major search engines: Google, Yahoo!, and MSN, were created to help webmasters specify which piece of content is unique, and ignore the rest.
Duplicate content is bad. When a search engine finds multiple pages with the same exact content, it has to choose which to show. It will only show one piece of content in it’s index, and that used to mean your print-friendly page or affiliate pages. Multiple links to the same content also dillute pagerank.
Until now, the only way to control pagerank and duplicate content were to use the rel=”nofollow” tag in your links that you didn’t want to be indexed. The major search engines recognize nofollow, but you can only do this on links on your website, not affiliate links or links from third party sites.
The canonical URL tag allows you to specifcy which link and page should be indexed. It’s more of a suggestion, but Google, Yahoo!, and MSN are all paying attention. As long as you only specify one canonical URL tag per unique piece of content, the search engines will most likely follow your recommendation. Google said on their blog: “It’s a hint that we honor strongly.”
Canonical URL tags can give you an edge over the competition. Very few (less than 1%) of e-commerce websites sculpt pagerank by speficying which pages pass pagerank (through nofollow or canonical links).