Sharks are heavy fishes, possessing neither lungs nor swim bladders (see fish). Their skeletons are made of cartilage rather than bone, and this, along with large deposits of fat, partially solves their weight problem; nevertheless, most sharks must keep moving in order to breathe and to stay afloat. They are good swimmers; the wide spread of the pectoral fins and the upward curve of the tail fin provide lift, and the sweeping movements of the tail provide drive. Their tough hides are studded with minute, toothlike structures called denticles. Sharks have pointed snouts; their crescent-shaped mouths are set on the underside of the body and contain several rows of sharp, triangular teeth. They have respiratory organs called gills, usually five on each side, with individual gill slits opening on the body surface; these slits form a conspicuous row and lack the covering found over the gills of bony fishes. Like most fishes, sharks breathe by taking water in through the mouth and passing it out over the gills. Usually there are two additional respiratory openings on the head, called spiracles. A shark's intestine has a unique spiral valve, which increases the area of absorption. Fertilization is internal in sharks; the male has paired organs called claspers for introducing sperm into the cloaca of the female. Members of most species bear live young, but a few of the smaller sharks lay eggs containing much yolk and enclosed in horny shells. Compared to bony fishes, sharks tend to mature later and reproduce slowly.

Only a small number of the predatory species are definitely known to engage in unprovoked attacks on humans. The largest and most feared of these is the great white shark, which may reach 20 ft (6 m) in length and is probably responsible for more such attacks than any other species. Other sharks reputed to be especially dangerous are the tiger and blue sharks and the mako. Sharks are extremely sensitive to motion and to the scent of blood. Swimmers in areas where dangerous varieties occur should leave the water quietly if they are cut; spearfishing divers should remove bleeding fish from the water immediately. In some places bathing areas are guarded by nets. A number of substances have been used as shark repellents, including maleic acid, copper sulfate, and decaying shark flesh, but their effectiveness is variable. An electrical repellent device, exploiting the shark's sensitivity to electrical fields, has been developed in South Africa. Sharks usually circle their prey before attacking. Since they seldom swim near the surface, an exposed dorsal fin is more likely to be that of a swordfish or ray than that of a shark.

Sharks, rays (including skates), and chimaeras together form the vertebrate class Chondrichthyes, the cartilagenous fishes (see Chordata). The sharks and rays form the subclass Elasmobranchii, and the sharks form the order Selachii.