Google, Yahoo! and Microsoft have come together in an attempt to reduce duplicate content in SERPs due to multiple URLs pointing to the same content.  The new tag named the ‘Canonical Tag’ allows you to define what the real address of a page should be by providing a META TAG in the HEAD section of a web page.

For example, say you are selling an item which belongs to several categories.
You may have all the following URLs pointing to the same content:

http://www.example.com/climbing-gear/black-diamond-headlamp
http://www.example.com/camping-gear/black-diamond-headlamp
http://www.example.com/products/black-diamond-headlamp

Google and other search engines don’t like this because they consider each page to be unique thus making it seem like you have duplicate content (which is bad!).

The Canonical Tag allows us to specify which page should be used as the proper URL in Google, Yahoo! and Microsoft’s SERPs.

Simply add this tag to the HEAD section of each of the category pages that point to “headlamp”:

<link rel=”canonical” href=”http://www.example.com/products/black-diamond-headlamp” />

For further information check out the Official Google Webmaster Central Blog.