Quahog is a term used to refer to two different species of hard-shelled clams. The first species is the quahog, which is also known as the hard-shell CLAM. It is a large, edible CLAM found in the Atlantic Ocean and is typically used for chowder or other dishes. The second species is the soft-shell CLAM, which is smaller and has a softer shell. Both species are harvested for food and can be found in markets and restaurants.