URL Canonicalization

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
But technically all of these urls are different. A web server could return completely different content for all the urls above. When a search engine “canonicalizes” a url, it tries to pick the url that seems like the best representative from that set.
Read more from Matt Cutts http://www.mattcutts.com/blog/seo-advice-url-canonicalization/
Frank McCown on url canonicalization http://frankmccown.blogspot.com/2006/04/url-canonicalization.html