Chowder is a seafood or vegetable stew, often served with milk or cream and mostly eaten with saltine crackers. Chowder is usually thickened with broken up crackers, but some varieties are traditionally thickened with crushed ship biscuit. New England clam chowder is typically made with chopped clams and diced potatoes, in a mixed cream and milk base, often with a small amount of butter. Other common chowders include seafood chowder, which along with clams includes many types of fish and shellfish; corn chowder, which uses corn instead of clams; a wide variety of fish chowders; and potato chowder, which is often made with cheese. Some people include Manhattan clam chowder as a chowder, but since it has no milk or cream and is tomato-based it may be considered more akin to a vegetable soup with clams.
The origin of the term chowder is obscure. One possible source is the French word chaudière, the type of cooking/heating stove on which the first chowders were probably cooked. (This, if true, would be similar to the origin of casserole, a generic name for a set of main courses originally prepared in a dish called a casserole.) Chodier was also a name for a cooking pot in the Creole language of the French Caribbean islands: Crab pas mache, li pas gras; li mache touop, et li tomber nans chodier (if a crab don't walk, he don't get fat, if he walks too much, he gets in a cooking pot).  Another possible (and maybe more probable) source could be the French dish called chaudrée (sometimes spelt chauderée) which is a sort of thick fish soup from the coastal regions of Charente-Maritime and Vendée.
The phonetic variant chowda, found in New England, is believed to have originated in Newfoundland in the days when Breton fisherman would throw portions of the day's catch into a large pot, along with other available foods.
Fish chowder, corn chowder, and clam chowder continue to enjoy popularity in New England and Atlantic Canada.