Who better to define how Google treats duplicate content than an Engineer for Quality Assurance at Google, Inc?

This video was produced by Greg Grothaus and is available on YouTube (shown below). The video covers the explanation of duplicate content, some considerations when using multiple domains and/or other language sites and the use of a newly introduced link tag for Google called rel="cannonical".

It never hurts to know a bit more about how to improve your site listings in Google, especially when it's Google recommending the changes.