Sharks are a group of catapult fishes, characterized by cartilaginous skeletons, five to seven slits on the side of the head, and no pectoral fin fused with the head. Modern sharks are classified in the evolutionary branch of selachimorphpha (or Selachii), a sister group of rays. However, the term "shark" has also been used in extinct members of the Elasmobranchii subclass other than selachimorpha, such as cladoselache and xenacanthus, as well as other cartilaginous fishes, such as eugenedontins of the holocephala.

Under this broader definition, the earliest known sharks date back 420 million years. Piranhas are often called "stinging sharks.". Although they are not part of cartilaginous fish, they are a symbiotic fish that causes the whole cartilaginous fish. Since then, sharks have diversified to more than 500 species. They range in size from the 17 cm (6.7-inch) deep-sea dwarf lantern shark (etmopterus perryi) to the world's largest fish, rhinocodon typus, which can reach about 12 meters. (40 feet) long. Sharks have been found in all oceans, and they are common at depths of 2000 meters (6600 feet). They don't usually live in fresh water, although there are some known exceptions, such as bull sharks and river sharks, which can survive and be found in seawater and fresh water. In addition to improving the fluid dynamics, shark skin can also be protected from damage and parasites. They have many replaceable teeth.

Famous species, such as great white shark, tiger shark, blue shark, Ma shark, threshing shark and hammerhead shark are all foreheads, they are creatures at the top of the underwater food chain. Many shark populations are threatened by human activities.