All of the milk on the shelves at your local grocery store has been pasteurized to ensure that it's safe to drink.

Pasteurization makes milk safer to drink by heating it and killing potentially dangerous germs or harmful microbes.

Microbes

Microbes are particularly a problem in the modern age because these days most milk is transported long distances and stored for longer than was the case a hundred years ago when people bought milk from local dairies and drank it almost immediately.  With transportation and storage, microbes have plenty of opportunity to cause milk to spoil.

Why Pasteurize?

Pasteurization kills most of these microbes, leaving milk that can be stored for longer periods of time.

A standard pasteurization process is to heat milk to 144 degrees Fahrenheit and maintain that temperature for thirty minutes.  This process does not change the flavor of milk much because it takes place at a relatively low temperature.

Another way to pasteurize milk is to heat it to 160 degrees for fifteen seconds. This second process produces a slightly less palatable milk that can be stored for longer periods of time because in those fifteen seconds at a higher temperature, more microbes are killed.

A final way of processing milk is to heat it to 280 degrees for one second, called ultra-pasteurization.  This method leaves a cooked flavor, but a final product that can be stored for longer; it is used mainly with cream because cream is used more slowly than milk, and needs to keep in your refrigerator for longer than a few days.