Curd is obtained by coagulating milk in a sequential process called curdling. It can be a final dairy product or the first stage in cheesemaking. The coagulation can be caused by adding rennet, a culture, or any edible acidic substance such as lemon juice or vinegar, and then allowing it to coagulate. The increased acidity causes the milk proteins (casein) to tangle into solid masses, or curds. Milk that has been left to sour will also naturally produce curds, and sour milk cheeses are produced this way.

Milk is a white liquid food produced by the mammary glands of mammals and the caecilian Siphonops annulatus. It is the primary source of nutrition for young mammals before they are able to digest solid food. Immune factors and immune-modulating components in milk contribute to milk immunity. Early-lactation milk, which is called colostrum, contains antibodies that strengthen the immune system and thus reduce the risk of many diseases. Milk contains many nutrients, including calcium and protein, but also lactose and saturated fat.