Crustaceans are one branch of the arthropod phylum, and like all arthropods, have segmented bodies and several jointed legs. Crustaceans have chitonous exoskeletons (shells, in normal parlance) which can be thick and strong or transparent and delicate. They are primarily aquatic, usually marine, but some freshwater dwelling.
The most familiar of the crustaceans to the non-marine biologist are probably edible shellfish such as crabs, shrimps, and lobsters, but in addition there are many microscopic crustaceans in plankton. There is also a terrestrial order of crustaceans colloquially known as woodlice or pillbugs; they live in damp areas and curl up into a ball when disturbed, relying on their exoskeleton for protection.