Scalding milk is bringing milk to just below a boiling temperature, then removing it from the heat. It is no longer needed these days: older recipes would have you do it before the days of pasteurization.
Some people, however, feel that scalding milk still serves a useful purpose — say, in bread recipes. They feel that scalding the milk breaks down a protein that can affect the texture of bread, and the filling in certain pies such as Pumpkin Pies. Milk contains serum proteins that could interact with flour proteins to form a weak dough. Scalding the milk (and then cooling before using, so you don’t cook your dough in the bowl) inactivates these serum proteins in the milk.
The skin that forms on the top of boiled milk is the protein in milk coagulating. When you skim this off, you are removing a great deal of the nutrients in the milk. The skin is caused by evaporation of water at the surface, which lets the protein molecules come together. You can help prevent this by covering with a lid, or whisking it just enough to produce a bit of foam. Whey protein particles fall to the pan bottom when milk is heated, where they scorch: so use medium heat or a double-boiler.