Butter and cheese are made of the same ingredient: milk. The main difference, however, is that butter consists of mostly milk fat. In fact, according to FDA standards, it must contain no less than 80% milk fat. Cheese on the other hand, consists mainly of milk protein. Its manufacturing process requires adding rennet, an enzyme that causes milk proteins to coagulate, or curdle. This curd is then pressed and processed into the cheese that we see everyday.