Milk is produced by female mammals to feed their young. That most commonly consumed by humans comes from cows, though goat's and other milks are also consumed.
Tips from the BBC "Milk scorches easily if heated to a high temperature. To reduce the likelihood of scorching, wet the bottom of the milk pan first. The water will help stop adhesion and minimise scalding. Milk forms a 'skin' when heated which, annoyingly, is impossible to stir back in once it's formed. To help prevent a skin forming, heat milk at the last possible moment, or whisk the surface to a light foam when heating."
These days all the milk we consume is processed in at least one way and often rather more.
Pasturisation In Australia all Milk must be "pasteurised by heating to a temperature of no less than 72?C and retaining at such temperature for no less than 15 seconds and immediately shock cooling to a temperature of 4.5?C" or an equivalent process. This kills unwanted bacteria. It also changes the nature of the milk. This is becoming a contentious issue particularly with the milk used for the manufacture of cheese.
Homogenised milk Milk's fat molecules are broken down is size so that they stay in suspension and don't separate out and settle on top of the milk. Most milk is homogenised today. There are some health concerns with this process.
Reduced fat and high calcium milk Milk has a fat content of less than 4% but even that tiny amount has become too much for those who care more for la bella figura than flavour. The levels of fat and calcium are manipulated during processing.
Other milk products are cream, with varying degrees of butterfat, cheese of all types and powdered and condensed milk.