This is pasteurized skim milk reduced to a powdered concentrate. It can be found in two forms, regular and instant. The instant variety has been given further processing to make it more easily soluble in water than regular dry milk. It takes 3.2 oz or about 3 tablespoons of instant nonfat dry milk added to 32 oz of water to make 1 quart of milk you can drink, bake or cook with just like fresh milk. The key ingredient is the dry milk so buy and store these products accordingly.
Dry buttermilk is for use in recipes calling for buttermilk.
It can be reconstituted into liquid buttermilk, but it's not much like the fresh liquid product and is best used in baked goods. Since it has a slightly higher fat content than nonfat dry milk, it generally does not keep as long.