For an updated version of this post, click here.
What is duplicate content you ask? When a search engine spider/bot finds multiple pages containing the same information, it is duplicate content. Sometimes this is done on purpose by 'black hat' SEOs, but sometimes it is just accidental because you just don't realize the engine sees 2 pages as being different URLs. This is usually the point when I would throw out one of my all time favorite words, URL canonicalization...but instead I will point you to a great visual representation of duplicate content by Ann Smarty at Search Engine Journal. Enjoy!