Google India have posted India Site Clinic - Part 1 which states much about this canonical link element . This is a simp,e thing but google says that Microsoft and Yahoo also will follow this.  This is actually a simple thing of pointing the website in different URLs, like http, https, https://www etc to single URL which will help Search engines to avoid duplicates.

http://www.example.com/
http://example.com/
https://example.com/
http://example.com/index.html
http://www.example.com/index.html

 

View a video on this from Google, Matt Cutts talking about canonical link element.

Read more here with examples and how to fix