Today, we made our own butter and we learned that butter is essentially the fat of the milk. Milk is basically made up of water and fat globules. These globules are very small, and form a stable suspension known as a colloid.
Fat is lighter than water so will float to the top of the milk and it can be collected as cream and has a lot more fat in it than normal milk. The globules still tend to repell one another so they still stay about the same size and can flow past each other easily. By whisking the cream, you are overcoming the repulsion between the globules and causing them to join up. Eventually the globules of fat become a similar size to the bowl you are using and the cream splits into areas of mostly water and areas of mostly fat. The mostly water part is called buttermilk and can be pressed out. The other part is mostly fat with a bit of water inside, and is known as butter. Because the fat is airtight, oxygen can't get into the butter and bacteria cannot grow easily. So, the butter you make will last much longer than the milk or cream which is why butter was originally made.
In the 19th century, butter was still made from cream that had been allowed to stand and sour naturally. The cream was then skimmed from the top of the milk and poured into a wooden tub. Buttermaking was done by hand in butter churns. The natural souring process is, however, a very sensitive one and infection by foreign micro-organisms is common.
Today's commercial buttermaking is much more complex. The commercial cream separator was introduced at the end of the 19th century, the continuous churn had been commercialized by the middle of the 20th century.
We just used whipping cream and a hand mixer. It took 30 minutes. Imagine how long it would have taken if we had been hand churning! Then, when we were done, we made cookies with our butter.
Most of our information came from http://www.foodsci.uoguelph.ca/dairyedu/butter
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