Think of buttermilk as slightly sour milk because it has curdled proteins. The thickness of buttermilk falls somewhere between that of milk and cream. It's also much lower in fat than milk or cream. This versatile ingredient can be used to make Southern buttermilk pie. It is also good for marinating chicken to keep it moist during cooking. In some European countries (looking at you, Holland!) people like to drink a glass of it with lunch instead of regular milk or soda.