Porpoises are smaller than dolphins and swim much faster. Porpoises have rounder faces. Dolphins and porpoises are actually both small whales.

Dolphins and porpoises are entirely different species belonging to distinct families within the toothed-whale (Odontoceti) suborder of the Cetacea. As mammals, of course, dolphins and porpoises all breathe air and bear live young. All dolphin and porpoise species possess teeth; a telescoped skull shape; the same up-and-down movement of their tails for swimming; a single nare (blowhole) at the top of the head for breathing; and some ability to emit sound either to navigate, locate their prey or to communicate with each other. Dolphins and porpoises also have a strong instinct for community. Dolphin and porpoise herds travel together for life. For all their similarities, they are still separate species. They possess similar biological characteristics, but their appearance differs according to genetics and their lifestyles as a result of their habitats and survival needs. On close observation, several obvious physical characteristics separate dolphins from porpoises. Probably the best way to tell dolphins and porpoises apart is by the shape of their heads. Dolphins tend to have a large, rounded head with a well defined snout while porpoises tend to have a smaller, rounded head and a very short, indistinct snout or beak. Another difference is the dorsal fin, which on dolphins is prominent, broad-based, crescent-shaped and pointed at the tip. It is small, triangular and blunt tipped on the porpoise. Their teeth are also different. The dolphin's teeth are rounded and cone shaped. The porpoises' teeth are flat and triangular. Dolphins are swift, acrobatic swimmers. They love to play and spend a good deal of each day doing so. Porpoises tend to be more serious and spend most of their time feeding. In addition, they tend to be much more wary and stay clear of people and boats. Reference Link: http://www.flseagrant.org/library/publications/fathom_magazine/volume-6_issue-1/dolphins_porpoises.htm

Also they have different shaped dorsal fins. A porpoise has a wedge shaped dorsal fin, while dolpins have more of a sickle shaped dorsal fin

For one thing, dolphins are more intelligient, of course. For another, dolphins have many un-fused vertebrates in their neck, allowing them to move their heads side to side, while a porpoise has fused vertebrates, therefore it has to turn completely around to see the the left or right. Also, dolphins are a more playful, and have a longer rostrum [mouth area] and dolphins are bigger. I hope that helps =]]