Cheese curds are the fresh curds of cheddar cheese. They are generally available in retail stores operated at cheese factories in Wisconsin (and perhaps in other dairy states) and throughout the province of Quebec in Canada.
They are little-known elsewhere, because they must be eaten absolutely fresh, within hours of manufacture. After twelve hours, even under refrigeration, they have lost much of their "fresh curd" characteristics, particularly the "squeak." After twenty-four hours, they have lost them entirely. They still taste amazing like all cheese, but they are not fresh cheese curds any more—they are just boring, insufficiently aged cheese. No ordinary shipping method is sufficiently fast to preserve their flavor intact.