A quahog is a kind of edible clam with a very hard shell. In the US, you're most likely to see quahogs on the menu in New England, New York, or New Jersey (not so much in Montana).
Quahogs are sometimes called "hard clams," "round clams," or "chowder clams," since they're commonly used to make clam chowder. Native Americans in New England once used quahog shells (especially purple ones) to make a kind of bead called wampum that was used as a gift or for trading. The word quahog comes from the Narragansett poquauhock.