Sharks, together with races, torpedos and chimeras, are fishes belonging to the class of the condroittis (Chondroichthyes) and they are present in all the seas of the globe, from the surface up to 1500 meters deep, from the warm tropical waters, to the colds arctic and Antarctic seas.
Sharks have started to swim in the oceans million of years before the appearance of the dinosaurs, but today these fishes are decimated by the commercial fishing that sets a lot of species to risk of extinction.
Unfortunately sharks are considered as murderous raiders that eat anything they meet, but reality is different.. These elegant cartilagineous fishes are only great raiders, almost to the vertexes of the food chain (they have few natural raiders: greater sharks and the orcas) and they are important to regulate and to maintain the equilibrium of the sea ecosystem. With the feeding in fact the sharks regulate the abundance, the distribution and the difference of their preys, they favor the natural selection eliminating the weak and sick individuals and they get sources of food for the scavenger animals.
Every year, in all the seas of the world,100 million sharks are killed, 20 million concern the blue shark (Prionace glauca).
Recommended reading: "Sharks, great raiders.. raided."

There are about 450 kinds of Sharks, an about fifty of which present in the Mediterranean, with dimensions that go from the only 24 cm of the Pygmy Shark from the ribbon tail (Eridacnis radcliffei) to the 18 meters of the Whale Shark (Rhincodon typus), that represents with these measures also the greatest fish actually known. (Complete sharks list)
The most ancient fossils of these animals go up to around 400 million years, and the biologist thinks that their evolution has even already arrived at the most level 100 million years ago.
The skeleton of the sharks is not bony, as that of the common fishes but cartilaginous, formed by cartilage, similar to that that constitutes the ear or the human trachea.
Contrarily of the common fishes, the sharks don't possess the swimming bladder (an inside afloat body that can fill him with gas) whose function of support to the floating is replaced partly from the big liver, that can reach the 25% of the animal weight. Sharks are predators and in their diet it can be present fish, smaller shark, shellfish, mollusc and sea mammal.
The biologist knows three kinds of sharks that eat plankton, filtering the water that enters from their jaws. They are the already quoted Whale Shark (Rhincodon typus, up to 18 m.) Elephant Shark (Cetorhinus maximus, up to 13 m.) and the Great mouth Shark (Megachasma pelagios, up to 5 m.).  Sharks form is generally tapered and hydrodynamic, with the lengthened face and pointed, to favor the advancement in water.  Visit SHARK-MAP and Shark anatomy to know more.