Size. Orcas are the largest dolphins. They have a complex social organization where they hunt Cooperatively to bring down prey. Some porpoises hunt similarly but many are relatively solitary. read more
The orca, or killer whale, for example, is actually the largest member of the dolphin family. Dolphins are by far more prevalent than porpoises. Most scientists agree that there are 32 dolphin species (plus five closely related species of river dolphin) and only six porpoise species. read more