The guys at Google have released guidelines that will allow users to publicly specify in code a prefered version of a url. A single webpage can sometimes have multiple url versions for a single page, this creates duplicate content which can have nasty effects on your ranking abilities. This is a God send for dynamic sites having hundreds of duplicate listings in Google and Yahoo.
Lets look at an example of a website selling Swedish Fish. Your prefered version of the page and url looks like this:
http://www.example.com/product.php?item=swedishfish
However, users (and Googlebot) can access Swedish fish through multiple (not as simple) URLs. Even if the key information on these URLs is the same as your preferred version, they may show slight content variations due to things like sort parameters or category navigation:
http://www.example.com/product.php?item=swedishfish&category=gummycandy
Now, you can simply add this <link> tag to specify your preferred version:
<link rel=”canonical” href=”http://www.example.com/product.php?item=swedish-fish” />
inside the <head> section of the duplicate content URLs and prefered url:
http://www.example.com/product.php?item=swedishfish&category=gummycandy
http://www.example.com/product.php?item=swedishfish&trackingid=1234&sessionid=5678
and Google-bot will understand that the duplicates all refer to the canonical URL: http://www.example.com/product.php?item=swedishfish. Additional URL properties, like PageRank and related signals, are transferred as well.
This will stop the duplicate content issue, and allow better indexing and surfing. Thanks Google.
There is word that Yahoo and Live Search will also adopt this new method.