Yogurt is made from a mixture of skim milk and condensed milk to get a higher solids concentration. These three items are considered cultured milk products because the addition of a bacterium is required to make them taste the way they do. The bacterium added reacts with the lactose in the milk and convert it to lactic acid. The lactic acid causes the milk protein, or casein, to coagulate or clump together. The type and amount of bacterium used give each of these items its unique flavor.
Cheese is another type of cultured milk product.
In this case, the starting material, milk, is mixed with rennet and bacteria or other microorganisms to help change the lactose to lactic acid. The curds are cut up and then lightly heated, causing them to shrink. Some cheeses can be eaten immediately, for example mozzarella. The whey can also be further processed to make cheeses like ricotta.