Shellfish is a culinary and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some kinds are found in freshwater. In addition a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean.
Despite the name, shellfish are not a kind of fish, but are simply water-dwelling animals. Many varieties of shellfish are actually closely related to insects and arachnids, making up one of the main classes of the phylum Arthropoda. Cephalopods and bivalves are molluscs, as are snails and slugs.
Familiar marine molluscs enjoyed as a food source by humans include many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans commonly eaten are shrimp, lobster, crayfish, and crabs. Echinoderms are not as frequently harvested for food as molluscs and crustaceans; however, sea urchin roe is quite popular in many parts of the world. (source : wikipedia)