Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Crocodilia
Family: Alligatoridae
Alligators belong to the order Crocodilia, which is made up of 20 large lizardlike species. They are the largest modern reptiles.
There are two living alligator species. The American alligator is found in the southeastern United States from North Carolina to Florida. The endangered Chinese alligator lives in China in the Yangtze River valley. American alligators live in freshwater, such as ponds, marshes, rivers, lakes and swamps.
So what is the difference between alligators and their crocodile cousins? The main difference is in their snouts. Alligators have wider heads and blunt snouts. The crocodile has a pointier snout. 
Alligators upper teeth can be seen when their mouths are closed, but their lower teeth cannot. With crocodiles, you can see both top and bottom teeth when their mouth is closed. Crocodiles can also grow much larger than alligators. Alligators can be up to 20 feet long, but six to eight feet long is average. They can weigh as much as 1,000 pounds! Crocodiles are usually more dangerous to humans than alligators.
Alligators spend most of their time in the water but can travel on land by sliding on their bellies and walking with their legs extended. When they float in the water, you can see only their nostrils, eyes and ears above the surface. They are nocturnal, so they are active mostly at night. During the day, they will lie in the sun beside the water or on logs in the water. At night, they look for their food, which includes tadpoles, frogs and fish. Older, larger alligators eat mammals, birds and other reptiles.
Sources: Alligator and Crocodile, Britannica Student Encyclopedia from Encyclopdia Britannica Online; Alligator, Wikipedia.
