Mozzarella is a white Italian cheese. Fresh mozzarella is soft, mild, and a little wet — it often sits in a small bag of water. When you heat it, it stretches into long strings, which is why pizza places use so much of it. The old, famous kind is made from water-buffalo milk (packs say di bufala); a lot of everyday mozzarella is cow’s milk, and the drier, rubbery stuff on frozen pizza is still mozzarella, just made to melt, not to eat in a salad. You also eat it cold with tomato and basil. Though actually, not every white pizza cheese is mozzarella, and “buffalo” here is the water buffalo, not the American bison.