Magento Canonical URL SEO extension is designed to limit duplicate content and also to improve pageRank.

It set one canonical url for some urls which would be considered like duplicate content.

Some examples

General

http://myWebsite.com/, http://myWebsite.com, http://myWebsite.com?SSID=123456789&fromSite=anotherWebsite have the same master url which would be : http://myWebsite.com

Pager and search

http://myWebsite.com?q=search&p=2&color=red have for matser url : http://myWebsite.com?q=search&p=2

(We keep pager and search parameters in the canonical url)

Pager first page

It takes care about the first page url generate by the pager : replace http://myWebsite.com/list?p=1 by http://myWebsite.com/list