Cream churned into a solid state. In the U.S. butter must contain at least 80% milk fat, and the USDA grades its quality on flavor, body, texture, color and salt (AA, A, B, and C). Sweet butter is simply made with sweet, as opposed to sour cream -- almost all. All butter is salted unless it specifically says unsalted, in which case it has absolutely no salt. Unsalted is preferred in baking so the baker has control over the salt content, but is more perishable (salt is a preservative). Whipped butter has air beaten into it, which makes it easier to spread when cold. Light or reduced-calorie butter has about half the fat as regular through the addition of other stuff (skim milk, water, gelatin, read the label).
Season: available year-round
How to select: Butter is perishable, check the pack date at the store before you buy.
How to store: Butter will pick up any kind of odor, so store air tight in the refrigerator, 1 month for regular and 2 weeks for unsalted, or both in the freezer for up to 6 months.
Substitutions: margarine, do not substitute whipped or low-fat butter when baking; 1 stick of butter = 8 tablespoons = 1/2 cup = 1/4 pound; To cut cholesterol and fat, you can also use oils in place of butter on a 1:1 ratio in some recipes, although texture and taste are often affected. This must simply be worked with.