Octopuses have a pouch-shaped body and eight powerful arms (not tentacles), usually with two rows of suction discs on each. They don't have any bones, but in some cases, the brain may be enclosed in cartilage. Their size can range from 1.5 cm (0.6 in.) to more than 5 m (16 ft.); spans of up to 9 m (30 ft.) have been recorded, although these measurements are often considered questionable. The arms usually make up about half their span.

Their skin has amazing abilities. Octopuses can change their colour to match the background and thus provide camouflage. They can change the colour pattern in order to communicate with other octopuses. They can also change the texture of the skin's surface to further help them blend with their surroundings. They inhabit many diverse regions of the ocean, especially coral reefs.

Octopuses also have a sac that contains an inky substance. When an octopus senses danger, it ejects some "ink". The ink darkens the water, thereby screening the octopus from the other animal. In some species, the saliva—deployed by a beak bite—can also paralyse prey.

Octopuses can travel through water by jet propulsion: they take in water and forcibly eject it through a funnel, using the contraction of their muscular mantle (the layer of tissue that encloses the body). They usually use this method only when trying to escape from danger or when catching prey. They are also able to creep over hard surfaces using their arms, and indeed, this is how most octopuses move around most of the time.

Octopuses are classified as molluscs. They are closely related to squids, cuttlefish and nautiloid. Other molluscs such as snails, mussels and clams are slightly more-distant relatives. There are around 300 recognized species; octopuses are found in all the oceans. Some species are edible, including the ink!