If you're looking at a used car, enter the vin number into Google. It will often bring up photos from any auctions it went through and you might see that it had been in a wreck and fixed without being reported to Carfax.
It's a reporting system for vehicles so that the next owners of a vehicle know its repair history. It will also tell you if any loans or leins are placed against the vehicle, so you don't get screwed. Car Facts.