Ancient relatives of sharks first appeared in the world's oceans about 400 million years ago. Today, sharks are classified among the group of fish called the elasmobranchs, which also includes rays and skates. There are well over 350 known species of sharks, and new species are discovered nearly every year.

Although sharks are fish, scientists distinguish between them and bony fish. Sharks and other elasmobranchs are cartilaginous fish, meaning that their skeletons are made of cartilage, not bone. Their skin is covered with denticles, toothlike scales that differ from the scales of bony fish. And they have five, six, or seven gill slits per side, not one per side like bony fish.

Sharks reproduce in one of three ways. Some sharks lay egg cases with developing embryos inside. These sharks are called oviparous (producing eggs that hatch outside the body). Other sharks carry young internally, nourished by a placenta before being born alive. These sharks are called viviparous (giving birth to live offspring). Finally, some sharks bear live young that developed internally within egg cases and without attachment to the mother. These sharks are called ovoviviparous (producing eggs that hatch within the female's body). Gestation lasts anywhere from nine months to as long as two years. Depending on the species, a female may produce anywhere from 1 to 100 young at a time.

The average life span of most species of shark is less than 25 years. One species, the spiny dogfish, however, is known to live as long as most humans: 70 to 100 years. Unlike many creatures, sharks continue growing for the duration of their lives.