Anthony's response is correct for explaining what a canonical does. My response is for how to implement it. Say you have an online store that uses a bread crumb system. While you may have one main URL for a widget, multiple URLs could be created if visitors are taking multiple paths to find this widget. So say the main URL for this widget's page is: example.com/widget You could have many copies of this page on your website with different URLs. If you have URLs such as example.com/widget/1, example.com/widget/2, example.com/widget/3, and they all have the same content as example.com/widget, Google will see all of these pages as being duplicate content. So to take care of this you use a canonical. If you want example.com/widget to be the page that has the authority over the rest of the other URLs with the same content, you will need to create a canonical. The canonical for example.com/widget is: SEE EDIT BELOW Then you will want to take the canonical and put it somewhere inbetween the header for all of those URLs that have the same content. And as Anthony said www.example.com/widget is considered a different page than example.com/widget, so it would need the canonical from above as well, and the same goes for www. example.com/widget/1-3. http://www.ginzametrics.com/cheatsheet This link will take you to a great tool that generates meta tags and can also create a canonical link for you, if you don't want to type it all out. To make a canonical with the tool just copy and paste the main URL that you would like to use and it will create the canonical link below that you can copy and paste into the the pages head. EDIT: I don't think SEOmoz will let you post canonicals. But if you go the the link with the tool I provided you should still be able to create a canonical. It is a very simple and straight forward tool that can generate the canonical for you. Good luck.
