Original: In England, curds produced from the use of rennet is referred to as junket, with true curds and whey only occurring from the natural separation of milk due to its environment (temperature, acidity).
Simplified: Curds are a dairy product made by curdling milk with rennet or yeast so that it separates, and taking away the liquid part (called whey). Milk that has been left to go sour will also naturally separate into curds and whey.