If raw milk is left standing for a long time, the cream will rise to the top. Today, our milk does not have cream on top. This is because it is homogenized. Homogenization breaks up the cream into very tiny particles and spreads them throughout the milk in a permanent suspension, so they cannot join and form a lump. This is done by passing the milk very quickly through a tiny opening or nozzle, which forces the cream particles to disperse.
Homogenization is handy because, it is difficult to shake the milk to re-mix the cream layer.
Drinking milk is homogenized, but not all dairy products are. For instance, butter and cheese are not usually made from homogenized milk or cream, but yogurt and ice cream are.