They're all members of the order Crocodilia, but that doesn't mean that crocodiles, caimans and alligators are the same animal. The easiest way to tell the difference between the two better-known species, crocodiles and alligators, is by taking a look at their heads: crocodiles have slender, V-shaped heads, while alligators have broader, U-shaped heads. Their teeth are also telling; when crocodiles close their mouths, both their top and bottom teeth are visible, but only the alligators' upper teeth are visible.
According to a recent study, there also may be a less obvious difference between crocodiles and alligators: the proportions of the bones in their forelimbs and hind limbs. The study suggests there may be slight differences in the lengths of these bones—it suggests that alligators have shorter humerus bones in the forelimbs and shorter femurs in their hind limbs than crocodiles—and these findings can provide more information on the way these two animals move.
Caimans, on the other hand, belong to the same family as alligators, so it's trickier to tell them apart. But to do so, you take a look at their—you guessed it—heads. Caimans have the same broad, U-shaped head as alligators, but have longer, thinner teeth than their alligator counterparts. As a general rule of thumb, these reptiles are typically smaller than both alligators and crocodiles, though adult black caimans can grow to be up to 14 feet long.
There's also a big difference in where you can find crocodiles, caimans and alligtors. There are two species of alligators, which can be found in the United States and China. Caimans are the tropical cousins of alligators, and can be found in Central and South America. Crocodiles have a much larger range, and can be found in all tropical areas of the world.
Alligators, crocodiles and caimans aren't the only members of the Crocodilia order, though. An Asian reptile called a gharial is also a crocodilian, and they can be found in India and Nepal.
See if you can spot the two species of crocodilians in the National Aquarium: a pair of Johnston's crocodiles in our Australia: Wild Extremes exhibit and two smooth-fronted caimans in Amazon River Forest!