Have a read through this (http://www.mattcutts.com/blog/seo-advice-url-canonicalization/) and you should have a better understanding.
Yep, that MC links should be sufficient to explain. In one line it is the problem with seach engines of treating /this-is-url and /this-is-url/ as 2 different urls
canonical url : For example, http://agriya.com http://www.agriya.com http://www.agriya.com/index.html http://www.agriya.com/default.html These URLs are referred same page . This is called canonical URL
little true !!! the URL you shown will considered as Different URL and Google will not consider it as same URL so it will be possible that http://agriya.com will rank top on your keyword and at same time your http://www.agriya.com will not rank. both with and without www. will be consider as canonical issue. you can visit this link (Matt cutts) to know about exact idea about this issue here http://www.mattcutts.com/blog/seo-advice-url-canonicalization/