They are from two different families. Crocodiles are from the crocodylidae family and alligators are from the alligatoridae family.
The easiest way to tell the difference between the two is by looking at their snout. The crocodile has a very long snout, while the alligator has a very wide snout. In addition, the upper and lower jaws of the crocodile are nearly the same width. The alligator on the other hand has a very wide upper jaw.
They also prefer different types of water. Crocodiles prefer brackish water. Brackish water is water that is a mix of fresh water and sea water. Estuaries and mangroves are typically brackish. Alligators prefer fresh water.
Alligators tend to be gray and black, crocodiles tend to be more of a tannish color. The snouts of alligators are broad, but thin on croc's. Alligators are only found in the southwestrn U.S., but crocodile's are found the world over. The alligators teeth are not visible when the mouth is closed but on croc's you can see the bottom teeth when the mouth is shut.
Alligators tend to avoid people and try to run away when confronted. Crocodiles are just the opposite and will advance towards human intruder's.
Some crocodiles can live in salt water, alligators can't tolerate salt water.
As a general rule alligators have a short wide jaw line while crocodiles have a long narrower jaw line. This is due to eating preferences. The wider jaw of the alligator enables it to crush prey such as turtles. The narrower jaw of the crocodile enables it to catch fish and mammals. Due to the structure of the crocodiles jaw, it teeth tend to stick out and are more visible.
Crocodiles can live in fresh water due to a gland in their on their tongue that helps them cope with salinity. Alligators, although they have this gland, are not as capable of dealing with salt water.
Crocodiles are much more aggressive than alligators. They are more likely to attack humans.
I certainly think it is appropriate that Bullgatortail answers this question for you. Primary differences include:
- They belong to two different taxonomic families.
- Alligators' heads are wider and shorter, with a more U-shaped snout (rather than the croc's V-shape).
- Crocodile jaws are the same width; 'gator jaws have wider upper jaws, and the lower jaw fits into it.
- Most crocs have a protruding front tooth.
- Crocs have a jagged fringe on the hind legs and feet; 'gators do not.
- 'Gators prefer freshwater; crocs prefer seawater. Both can live in either waters, however.
- 'Gators are usually darker in color than crocodiles.