YOgurt n curd are the same. Yogurt and curd are dairy products. Yogurt, though, results from milk fermentation---converting a substance into acid under anaerobic conditions---whereas curdling, or coagulation, is when milk sours or is treated with either enzymes or edible acids like lemon juice. Lactose, the sugar in milk, becomes lactic acid when it ferments. Lactic acid gives yogurt its distinct tangy flavor. Yogurt's health benefits enhance its popularity. Curd makers often employ the enzyme rennet, other bacteria or the acid vinegar.