Polarized Lenses – What are they?

If you’ve looked into getting a new pair of sunglasses, whether for sport or fashionable eyeglasses, no doubt the option of polarized lenses has come up.  But exactly what are they? How do they work, and what benefits do they provide over regular sunglasses?  http://www.zennioptical.com/500318-acetate-frame-with-polarized-magnetic-snap-on-sunlens.html