What is the difference between an alligator and a crocodile?

Alligators and crocodiles have many similarities, including both having short legs, powerful tails, and long snouts, but there are also a number of ways to tell them apart.  First, they are usually found in different locations, with alligators preferring freshwater areas and crocodiles being found in saltwater habitats.  Second, their snouts are shaped differently, with alligators having a broad, U-shaped snout, where the snout of crocodiles is more narrow and V-shaped.  You can often see some of the bottom teeth on a crocodile, even when its mouth is shut, but not on an alligator.  Finally, alligators usually appear to be more black or dark gray, with crocodiles appearing more tan or olive.