Whole milk is fresh milk as it comes from the cow, with nothing removed and nothing (except vitamin D) added. It contains about 3½% fat (known as milk fat or butterfat), 8½% nonfat milk solids, and 88% water.
Skim or nonfat milk has had most or all of the fat removed. Its fat content is 0.5% or less.
Low-fat milk has a fat content of 0.5 to 2%. Its fat content is usually indicated, usually 1% and 2%.
Fortified nonfat or low-fat milk contains added substances that increase its nutritional value, usually vitamins A and D and extra nonfat milk solids.
