First, lets start with the definition of Canonicalization courtesy Wikipedia:

"In web search and SEO, canonicalization refers to web content that has more than one possible URL. Having multiple URLs for the same web content can cause problems for search engines - specifically in determining which URL should be shown in search results."

Matt Cutts gives a some more information:

"Canonicalization is the process of picking the best url when there are several choices, and it usually refers to home pages. For example, most people would consider these the same urls:
• www.example.com
• example.com/
• www.example.com/index.html
• example.com/home.asp "

So, you should consistently use one URL (http://www.example.com OR http://example.com) so that the search engines do not misinterpret multiple listings as different URLs.

As far as duplicate content affecting PageRank, Matt Cutts has this to say:

"The only way someone would notice their PageRank changing last week is for example if they were checking for a different canonical url (e.g. externally visible PageRank is shown for www.example.com, but Google changes the canonicalization from www.example.com to example.com)."

hope that helps. if not could you clarify your question
Source(s):
http://www.mattcutts.com/blog/seo-advice-url-canonicalization/
http://www.mattcutts.com/blog/canonicalization-update/
http://en.wikipedia.org/wiki/Canonicalization


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