A crocodile is a large amphibious reptile. It lives mostly in large tropical rivers, where it is an ambush predator. One species, the Australian saltie, also travels in coastal salt water. In very dry climates, crocodiles may aestivate and sleep out the dry season. 

The modern type of crocodile appeared first in the Eocene period. Its ancestors go much further back to the Upper Triassic. The name "Crocodile" is also used for any member of the order Crocodilia. They are basically Archosaurs, a group which also includes the dinosaurs. There are many species of Crocodiles including the American, slender-snouted and Orinoco crocodile.

The crocodile can quickly snap its jaw shut with much power. However, crocodiles have little strength when opening their jaws, such that a person could hold the jaw shut with their hands.

Crocodiles range in size from African dwarf crocodiles that measure rarely over 5 feet (1.5 m) to saltwater crocodiles which can approach 23 feet (7 m).

Description 
Crocodiles live in rivers, lakes and dams in parts of America, Asia, Africa and Australia. Some of the crocodiles from Australia live in salt water. These saltwater crocodiles are normally bigger than the ones that live in fresh water. While crocodiles spend most of their time in water, they can come out and move around on the land. 

Crocodiles cannot breathe underwater: they breathe air, just like people. When they are not active, they can hold their breath for a maximum of about two hours underwater before drowning. Normally, when underwater, they are active and can hold their breath for a maximum of 20 minutes to one hour.

Behavior

Feeding
The crocodile looks like it is a log, and hides beneath the surface of the water. When an animal comes down to the water to drink, the crocodile lunges out of the water, grabs the animal in its jaws, and pulls it underwater, where it drowns. After the animal is dead, the crocodile will grab a piece of meat in its jaws and spin around in a "death roll".

What do they look like

Their colors range from brown to grey and have different patterns covering them.They have many shapes and they differ in color. They have sharp claws and teeth. They can also be a greenish-brownish color.Crocodile tongues are not free. They are held in place by a membrane that can not move. Crocodiles are unable to stick out their tongues.

Alligators and crocodiles 
Although there is not much difference in their life-style, biologists put alligators in a separate family. Gharials are also in a separate family, and Caimans are a sub-family of alligators.

The  difference between an alligator and a crocodile is that one can not see the fourth tooth in the lower jaw of an alligator when the alligator's mouth is closed. One can see the fourth tooth in the lower jaw of a crocodile when its mouth is closed.

Ancient crocodiles 

According to scientists, some ancient crocodiles walked on two legs like dinosaurs.  The scientists found preserved footprints in South Korea.  They found hind foot prints but no front foot prints or marks from a tail dragging behind.  

They were able to tell that these ancient crocodiles walked on their hind legs with their front legs and tails off the ground.  They used the whole foot, the way a human or bear does, not just the toes, the way a dinosaur, bird or dog does.

Species
Sixteen extinct species of crocodile have been discovered. They are:
 American crocodile
 Borneo crocodile
 Central African slender-snouted crocodile
 Cuban crocodile
 Dwarf crocodile
 Freshwater crocodile
 Hall's New Guinea crocodile
 Morelet's crocodile
 Mugger crocodile
 New Guinea crocodile
 Nile crocodile
 Orinoco crocodile
 Saltwater crocodile
 Siamese crocodile
 West Africa crocodile
 West African slender-snouted crocodile

References 

 Crocodilian Biology Database

Other websites

 Crocodile (Wikiversity)
 Saltwater crocodiles in freedom: Authentic pictures and descriptions
 Supersize Crocs - an interactive guide to crocodiles from PBS 
  Man shares bungalow with crocodile. BBC News.

Crocodilia
Living fossils
Reptiles