Sharks are marine animals that swim in all the oceans.
Among the 465 species of sharks, the majority of them appeared in their definite form some 100 million years ago.
These elongated fishes have a cartilaginous skeleton and respire through several gill slits. They range in size from 15 cm (dwarf shark) to 18 meters ( the whale shark).

A Shark's jaws move independently one from the other. They both contain at least two rows of teeth, that are replaced constantly in the course of the animal's life. Some hundred teeth as sharp as a knife's blade, constitute the principal weapon for a shark to attack a prey.

Sharks may reveal to be super-predators. Whereas some of them live on plankton, some others, like the white shark or the tiger shark, only like the taste of fresh blood.
A predator's instinct that sharks also express when they reproduce: the male bites violently the female in order to hold “her” down.

The human flesh is too low in fat to tickle a shark's taste bud. The probability to die from a car accident in Australia is 3000 higher than that of dying from a shark's bite. Except for the 'ocean's bin', the tiger shark, the majority of these animals only bite to have a taste of a prey. If the flavor-test reveals to be non-interesting, they usually release their hold.
A situation like that of the movie 'Jaw' is not really likely to happen. Surfers who swim away from the reef to chase a perfect wave may face a greedy shark, but have less chance to die from this unfortunate situation than an explorer who comes across a cobra.

A tip for you if you cross the way of a shark: unfold your legs or try to frighten him by swinging a long object under its nose.