The first difference between a crocodile and an alligator is that they are from different families of crocodilians. Crocodiles are from the crocodylidae family, while alligators and caiman are from the alligatoridae family.

In terms of physical differences the easiest way to tell the difference between the two is that a crocodile has a very long, narrow, V-shaped snout, while the alligator's snout is wider and U-shaped. Because of the wide snout of the alligator it packs more crushing power to eat prey like turtles that constitute part of its diet. The narrow crocodile snout, although still very powerful, is not really suited for prey like turtles but is very versatile for fish and mammals.

Another physical difference between the crocodile and the alligator is that the crocodile's upper and lower jaws are nearly the same width, so the teeth are exposed all along the jaw line in an interlocking pattern, even when the mouth is closed. They also have an enormous 4th tooth on the lower jaw that is accommodated by depressions in the upper jaw just behind the nostrils.

An alligator, on the other hand, has a wider upper jaw, so when its mouth is closed the teeth in the lower jaw fit into sockets of the upper jaw, hidden from view. Only the teeth of the upper jaw are exposed along the lower jaw line. Even the enormous 4th tooth on the bottom jaw, which is exposed in a crocodile, is hidden in the alligator.

Another physical difference is that crocodiles have a lighter olive brown coloration, while alligators appear blackish. Alligators also prefer freshwater while crocodiles like brackish water and sometimes even ocean.

While alligators (and caimans) are often confused with crocodiles, they belong to two quite separate taxonomic families.

The most obvious external differences are visible in the head-alligators and caimans have wider and shorter heads, and a more U-shaped than V-shaped snout. The alligator's upper jaw is wider than its lower jaw, and the teeth in the lower jaw fit into small depressions in the upper jaw. The upper and lower jaws of the crocodiles are the same width, and teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed. When the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define a species. However, in captivity, alligators and caimans may show jaw deformities which result in lower teeth protruding.

Alligators lack the jagged fringe which appears on the hind legs and feet of the crocodile and have the toes of the hind feet webbed, not more than halfway to the tips. Alligators strongly prefer freshwater while crocodiles can better tolerate seawater due to specialized glands for filtering out salt. However, both taxa can survive in either.

Both species of alligator also tend to be darker in color than crocodiles-often nearly black (but color is very dependent on water quality). Algae-laden waters produce greener skin, while Tannic acid from overhanging trees can produce often darker skin.

When cleaning alligator pools, some zookeepers can tread on alligators without eliciting a response, though crocodiles almost invariably react aggressively and are for the most part more aggressive in their natural habitat

In addition to the other answers given Crocodiles are also generally more aggressive without any provocation. Alligators are usually only aggressive without provocation during mating season in the spring. I would not trust their behavior at any time,and therefore avoid them always.
The shapes of their heads are different.

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