Dolphins and porpoises are both cataceans, i.e. marine mammals, and are closely related to whales.
Dolphins belong to the family Delphinidae and are up to 30 feet long. Porpoises, belonging to family Phocoenidae are smaller, but stouter than dolphins.
Dolphins have a bulbous "melon" in the head distinct beak with conical teeth. Their dorsal fin is curved.
Porpoises have small, rounded heads and blunt jaws with flat, spade-shaped teeth. Their dorsal fin is triangular.
edit Social Behavior
Dolphins are social, living in pods of up to a dozen individuals. In places with a high abundance of food, pods can merge temporarily, forming a superpod, which may exceed 1,000 dolphins. They communicate using a variety of clicks, whistles and other vocalizations, and ultrasonic sounds for echolocation. Dolphins can establish strong social bonds. They will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed.
Porpoises are relatively shy and are seen alone or in groups containing a few individuals, but rarely in large pods. These groups are not stable associations, but ephemeral groupings that change as individuals join or leave. Porpoises act in a coordinated fashion even when they are hundreds of feet apart. They are capable of echolocation for finding prey and group coordination, but do not use it to communicate as often as dolphins do.
Dolphins and porpoises are highly intelligent. They have large, complex brains and are self-aware like humans. They are among the few species in the world that can identify themselves in a mirror. After years of research, scientists have said that cetaceans (including porpoises, dolphins and whales) should have rights just like humans. Bottlenose dolphins call out the specific names of loved ones when they become separated. Other than humans, dolphins are the only known species to call each other by name.