Asif Anwar is a Search, Social, & Internet Marketing Consultant/Specialist working in Bangladesh (In Indian Subcontinent aka South Asia). Professionally consulting and helping small businesses on Search Engine Optimization (SEO) since 2003 and actively learning about Search Marketing trends from 2001. Passionate about Search Engine Marketing (SEM), Social Media Marketing (SMM), and Pay Per Click (PPC).
Like rel=”nofollow” URL tag, a new rel Tag (canonical) has been introduced and currently being supported by major search engine to tell the search engines that the specific URL is the duplicate.
<link rel="canonical" href="http://www.example.com/printed-version.html">
This tag makes the webmasters free from relying with server end 301 redirect management, since this URL tag will be work as same as 301 redirects. And this can help retain the Authority of the home or any other page by disregarding pages that support session IDs, affiliate versions, printed, pdf versions, etc. that create duplicates of the original page.
More FAQ on this is on SEOMoz.

See what Matt Cutts, the Web Spam Head in Google has to say about it:
© Asif Anwar of SEOPPCSMM.COM - Source: Rel=“Canonical” URL Tag Introduced by Google, Yahoo, and Live