Clams are shelled marine or freshwater molluscs belonging to the class Bivalvia. The term "clam" has no taxonomic significance in biology, but is often used to refer to any bivalve (a mollusc whose body is protected by two symmetrical shells) that is not an oyster, mussel, or a scallop, and that has a more-or-less oval shape. An exception is the razor clam, which has an elongate shell that suggests an old-fashioned straight razor. Clams can live to be up to 150 years old -- or perhaps longer (science suspects that some larger quahogs found off the East Coast of the US are as old as the Republic).
Clams can be hard-shelled or soft-shelled, according to the degree of calcification of their shells, according to species. They are eaten raw, steamed, boiled, baked or fried, again (often) according to species. Clam chowder is a popular soup in which clams figure strongly.