Canonical link element is usually the most misunderstood topic so here I'm sharing a video by Matt Cutts explaining canonical link element in detail http://www.mattcutts.com/blog/canonical-link-tag-video/
Very nice link, for those who don't know who Matt Cutts is, he is pretty much the "gamemaster" of SEO, controls what is effective and what is outdated in the SEO algorithms. Definitely check this out if you are interested in SEO