Louis Pasteur invented the process of "pasteurization" around 1865. Pasteurization involves heating something up to a specified temperature, and then rapidly chilling it again to kill microorganisms like bacteria that can live in milk. There are different standards in different countries and for certain foods, but a common process heats milk up to 71.7 degrees Celsius for 15-20 seconds (this is cooler than boiling water). Pasteurization has revolutionized the safety of dairy products. Unpasteurized milk can harbor serious pathogens, like Coxiella burnetii and Listeria monocytogenes, two different types of bacteria that can cause serious infection and even death in some people. These are especially dangerous to children.
"Raw milk" fans insist that unpasteurized milk is healthier for you because pasteurization destroys essential micronutrients in the milk, and that pasteurization denatures the protein in the milk. Denaturation is simply a process in which proteins lose their naturally folded states. This happens anyway when proteins hit the high-acid environment in your stomach! This happens every time you cook meat, or eat meat, or eat anything with protein. Your body must "denature" it and break it down into individual amino acids (the building blocks of protein) to use it! They are still protein, and they are still processed as protein in your body.
As for these micronutrients, pasteurized milk still contains the same amounts of potassium, calcium, and vitamin D as it did before. Milk may contain other vitamins which may or may not be affected by pasteurization, there is still ongoing research, but to me that is beside the point. Cow's milk is designed to nurture baby cows. Period. It is not made for us. We drink milk to get protein, calcium, potassium, and vitamin D, not to ingest all the small nutrients that are designed for baby cows! If you think that pasteurized milk is not good for you, then don't drink milk. Do not drink "raw milk," which may contain life-threatening bacteria, just to get a few small amounts of nutrients which you could get safely elsewhere! Instead, eat some vegetables, for goodness sake.