Sharks along with most other chondrichthyan fishes, such as stingrays, electric rays, skates, sawfish and guitarfish, are collectively known as elasmobranchs. A shark's skeleton is made mostly of cartilage than can be strengthened by deposits of minerals in areas subjected to special stress such as the jaws and vertebrae. Cartilage is an ideal tissue for sharks for several reasons. First, cartilage is lighter than bone, important for sharks which have no swim bladder.
Most bony fish utilize a gas-filled swim bladder for buoyancy.
Second, cartilage is a relatively flexible material, giving tensile force to swimming and turning movements. Many sharks can virtually turn on a dime, but their skeletal structure and fins are much less elaborately articulated than in bony fish. This combined with the lack of a swim bladder means that the bodies of sharks are on the whole less maneuverable than those of many bony fish. Lastly, cartilage can grow throughout the life of a shark. This streamlined, cylindrical body shape reduces drag and requires a minimum of energy to swim.