Starfish typically have five arms, or rays, but there are species with as few as three or as many as fifty. Arms occur in a variety of sizes and shapes, depending on the species. The sea bat (Patiria miniata), which is found from Alaska to Mexico, has webbed arms.

Starfish arms are multipurpose appendages, used for feeding, locomotion, smelling, tasting, vision, and reproduction. The bottom of each arm is lined with rows of tube feet that allow it to creep in any direction or cling to steep surfaces. Tube feet are also used for chemoreception (smelling and tasting) and, in primitive species of starfish, for sweeping food into the mouth. Although starfish don't have true vision, they do have an eyespot on the tip of each arm that allows them to distinguish between light and dark. Each arm contains sex organs used for sexual reproduction and in some species each arm, or even a piece of an arm, is capable of regenerating an entirely new starfish.

Starfish, also called sea stars, are perhaps one of the most familiar of marine organisms and are practically a symbol of ocean life. Despite their name, they are echinoderms not fish and breathe through structures on their skin, not through gills.

There are at least 1800 known species of starfish and they occur in all the Earth's oceans (never in freshwater). The greatest variety of species is found in the northern Pacific, from the Puget Sound to the Aleutian Islands. These bottom dwellers play a crucial role in the ocean ecosystem, as prey when they are free-floating larvae and as predator when they reach adulthood. Few animals eat adult starfish, which are apparently neither palatable nor nutritious.