In order to resolve issues often experienced by dynamically generated websites, the three big search engines have announced a new meta-tag (link tag) which allows a website to specify the URL to be returned in the search engine results.

Google gives the example of a site where the same page is accessible through two different URLs

http://www.example.com/product.php?item=swedish-fish and
http://www.example.com/product.php?item=swedish-fish&category=gummy-candy

Placing this tag in the header of both pages

< rel="canonical" href="http://www.example.com/product.php?item=swedish-fish">

Tells the search engines which URL to use.

Read more from Google here
and Yahoo here.

Microsoft gives examples of four URLs which can all be resolved to one with the new tag.

http://mysite.com
http://www.mysite.com
http://mysite.com/default.aspx
http://mysite.com/default.aspx?promo=ABC