Octopuses are cephalopods, related to squid.
It is common to sell octopus whole, frozen into a solid chunk of ice or fresh. Small octopuses are sometimes served whole or as hand-like halves with about 4 tentacles. Large octopuses may be sliced thin for sushi. Live octopus is sometimes eaten for the thrill of having the suction cups grab at the eater's mouth.
Ways to serve octopus include: