This question is paramount because they are similar in the water and many times people confuses them. Even more, some people sometimes use the word dolphin and porpoise interchangeably. However, this is a mistake, as dolphins and porpoises are in fact different animals.
While both are mammals and nurse their calves similarly, both have lungs and breathe air; they have several significant differences.
Taxonomically both are cetaceans and members of the Odontoceti suborder, but they belong to different families. Porpoises are part of the Phocoenidae family while dolphins belong to the Dolphinae family.
Physically, porpoises are smaller than dolphins, reaching up to seven feet long while dolphins reach up to ten.
Porpoises have small rounded heads while most dolphin species have a round bulbous in the head.
The dorsal fin of porpoises is different than the dorsal fin of dolphins, while the former have a triangle shaped fin, the dolphin’s fin is slightly curved backward.
Regarding their behavior, porpoises are shy creatures, which usually do not approach boats while dolphins show less fear, approaching more often to people.