Like so much seafood, clams have a lot of names that can be confusing. The same species can have different names depending on its size, for example. Or some bivalves, just because people like to steam them, are all called "steamers" even though they are different species. Here is a quick and simple guide to getting started so you know what you're buying and how to cook it.
Hard-Shell Clams / Quahogs / Round Clams
Hard-shell clams (Mercenaria mercenaria) go by many names. Littlenecks, topnecks, cherrystones, chowders - they are all the same clam, just different sizes (listed here from smallest to biggest). They live in the Atlantic Ocean along the East Coast of the U.S. and Canada in intertidal areas burrowed in the sand. They are harvested by simply digging them up. Smaller ones are eaten raw, shucked and on the half-shell whereas bigger ones are the basis of famed clam chowder. See below for more details about the different sizes.
Soft-Shell Clams / Steamers / Ipswich Clams
Soft-shell clams, commonly called steamers or Ipswich clams, are the species Mya arenaria. You can tell them from hard clams by their lighter colored, more oblong shells, which are also quite brittle and thus require some care when handling. They live in tidal flats on the eastern shore of Canada and the U.S. as well as across the Atlantic in the U.K., where they are also known as Essex clams. Because they live in the sand, they are famously gritty - in New England and other parts of the Northeast U.S. they are often steamed and then served with the resulting broth. To eat one pulls the clam from the shell, dips it in the broth to rinse any sand or grit off of it, and then might dunk it in melted butter before popping in one's mouth. They're also delicious fried or used in chowders.
Manila Clams / Steamers
Manila clams (Venerupis philippinarum) are, like Atlantic soft-shell clams, often called "steamers," since steaming them and pulling them from their shells to dip them in melted butter is a popular way to eat them. Originally from the shores of China and up to Siberia, they now grow on the West Coast of North America, too, where they are farmed along with other bivalves like mussels and oysters. You can learn more about manila clams here.
Razor Clams / Pacific Clams
Razor clams (Siliqua patula) are popular in Oregon and Washington. They have long, thin shells and plant themselves in the sand vertically. Razors need proper cleaning to make sure they aren't sandy or gritty when cooked up. How to cook them? The most popular way is to coat them in breadcrumbs and give them a quick dunk in the fryer. They can also be sauteed or broiled with great success.
Surf Clams / Bar Clams / Skimmers
Surf clams (Spisula solida) live on the Eastern Coast of the U.S. and Canada from South Carolina up to Nova Scotia. Beach goers on the Atlantic shore know the shells well - the surf tends to bring them to shore. It's also famous when sliced up, fried, and served as "clam strips." It's also a great clam to use for chowder.
Atlantic Jackknife Clam / Bamboo Clam / Razor Clam
Sometimes also called razor clams like those that grow on the West Coast of North America, Atlantic jackknife clams (Ensis directus) are long and skinny and burrow vertically into intertidal beach areas. You can read more about them here.
The smallest of the hard-shell clams (Mercenaria mercenaria), measuring just over one inch across (measured at the hinge). These are the clams most often seen raw, on the half-shell on seafood platters. Want to shuck them yourself? They're shucked just like oysters.