There are two basic varieties of clams – Hard Shell Clams and Soft Shell clams.
Soft shell clams found in the Atlantic include the ubiquitous steamer. Other East Coast hard shell clams include littleneck (<2 inches), cherrystone (a little bit larger at about 2 1/2 inches) and the quahog, also known as the large or chowder clam (3 inches or more).
The most notable west coast hard shell clams are the pismo found on Pismo beach. Small butter clams are found further north in the Puget Sound area are small and very tender clams. Soft shells found on the west coast include the razor clam and the geoduck clam (pronounced gooeyduck). Geoducks are a large six inch clam that can have a neck as long as two feet.