Dolphins, porpoises and whales are marine mammals belonging to the order Cetacea which comes from the Greek word ketos, meaning “large sea creature”. Dolphins are by far more prevalent than porpoises. Most scientists agree that there are 
32 dolphin species (plus 5 closely related species of river dolphin) and only 6 porpoise species. Collectively, whales, dolphins & porpoises are known as Cetaceans and are divided into 2 groups:
Group #1: Baleen whales are the “great whales” and as their name suggests, they all have baleen plates that are used to filter their food which consists of plankton and small species of fish.
Group #2: Toothed whales otherwise known as odontocetes and including all species of dolphin & porpoise 
which as you would expect, have teeth and eat larger prey items including at times, other marine mammals. The main differences with porpoises, are that they are usually smaller than other toothed whales and instead of cone-shaped teeth they have flat, spade-shaped teeth.
As a general rule, baleen whales are larger except the Sperm Whale and slower except the Fin/Finback Whale which is known as the “greyhound of the sea” than toothed whales. Additionally, all baleen whales have 2 blowholes, while 1 blowhole for toothed whales.
Difference between a dolphin and a porpoise is size, with all 
species of porpoise being that much smaller than their dolphin cousins. Porpoises don’t have the pronounced beak that most, but not all dolphins have and they also have a different teeth shape. Porpoise teeth are spade-shaped while dolphins are conical. A dolphin has a hooked or curved dorsal fin except for those species that don’t have a dorsal fin. A porpoise has a more triangular dorsal fin and generally speaking, dolphin bodies are leaner, while porpoises’ are a little more chunky.
Dolphins are also 
more “talkative” than porpoises. The whistles made by dolphins are produced through their blowholes and while porpoises do not do this, possibly due to structural differences in the porpoise’s blowhole, they can still be pretty noisy as they “puff” the air out when they surface. Dolphins & porpoises also have many similarities, one of which is their extreme intelligence. As research evolves, it is likely that more or perhaps fewer differences between dolphins & porpoises will be revealed.