Butter is in fact derived from milk. The milk is churned until it fully separates and becomes thicker. If the milk that is being used in this process is quite creamy, the butter will start to thicken quicker. Many people still make their own butter however it can also be purchased from just about any supermarket you walk into. There are many different varieties of butter and so it is important to select the kind that suits your needs specifically.
Those who are on a diet however might want to go for the fat free butter because of the fact that regular butter contains so much fat, around 80% actually. While this may be true, butter does not contain any trans-fat or carbohydrates which are widely unacceptable in most diets where the goal is to lose weight.