We were wondering if there is a difference between a dolphin and a porpoise or if they are just different names for the same animal.

They are two different, but closely related, groups of animals. Dolphins are in the family Delphinidae, which contains 30+ species. Porpoises are in the family Phocoenidae, which contains 6 species.

Both families are small toothed whales.

Dolphins have cone-shaped teeth; Porpoises have spade-shaped teeth.
Dolphins usually have a hooked or curved dorsal fin; Porpoises usually have a triangular dorsal fin.Dolphins usually have a beak; Porpoises never have a beak.


Since you asked about orcas also, I should note that orcas are dolphins.

Also, whereas porpoises and the sperm whale vocalize by producing clicks, dolphins, besides making clicking sounds also produce pulsed squeals. This is not, however, a characteristic used to establish the two families, as is the difference in their teeth.