First we heat the butter and skim off the froth, then we pour off the butter fat from the milk solids or white stuff in the bottom of the pan. Actually, the upper and lower layers are a combination of water, proteins and carbohydrates. Essentially the same in makeup, the top layer floats because it has air trapped within the froth and the bottom layer sinks because it doesn't. To make ghee, a type of butter used in India, let the milk solids burn a little before removing them. That gives the butter, or ghee, a slight nutty flavor.
Compound butter is an emulsion.
An emulsion is, by definition, a uniform mixture of two liquids that do not normally mix. An example is water and oil. The name comes from the Greek word for pearl, margaron. Margarine today is made largely of soybean or other vegetable oils combined with milk products and natural or formulated chemicals. Sauce pan, kitchen spoon, small bowl.