Google:Use of rel=canonical meta tag

If you have a single product page that can have multiple urls with slightly different parameters, that’s a great time to use a rel=canonical meta tag. You can use rel=canonical for pages with session IDs in a similar fashion. What rel=canonical lets you do is say “this page X on my host is kinda of ugly or otherwise isn’t the best version of this page.

Use url Y as the preferred version of my page instead.” You can read about rel=canonical at http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=139394. Bear in mind that if you can make your site work without session IDs or make it so that you don’t have multiple “aliases” for the same page, that’s even better because it solves the problem at the root.

Quote From :
mattcutts