They are cousins but belong to different families. All are members of the cetacean order, which includes whales, and all belong to the suborder odontoceti, literally meaning toothed whales.
The thirty-two species of seagoing dolphins (members of the family Delphinadae) are perhaps most easily distinguished from the six species of porpoises (members of the family Phocoenidae) by the fact that most dolphins have a well-developed beak and a bulbous forehead.
Dolphin bodies also tend to be more slender and streamlined than the whalelike porpoises. River dolphins in the family Platanistidae, which inhabit muddy rivers, have even longer, more slender beaks, prominent forehead pads, and small, weak eyes.
The bulging forehead of the dolphin houses a pad of fat called the melon, which is thought to help focus the high-frequency sonar beams with which dolphins navigate and find their food.
Orca, the killer whale of motion picture fame, Orcinuo orca, is actually a kind of dolphin, but one that lacks a pronounced snout. Performing dolphins are Atlantic bottle-nosed dolphins, Tursiops truncatus.