You may also be interested in Cookbook:Margarine.
Butter is a dairy product made by churning fresh cream. It consists of an emulsion of water and milk proteins in a matrix of fat, with over 80% being fat. It is used as a condiment and for cooking in much the same ways as vegetable oils or lard.
It is solid but soft at room temperature, and melts easily. Its color is generally pale yellow, but can vary from deep yellow to nearly white depending largely on what type of food the animals were eating. (Butter is typically paler in the winter, for example, when dairy cattle feed on stored hay rather than fresh grass).
It is easy to make your own butter in small batches and often worthwhile when serving butter on the table, such as with fresh bread. Take a small clean jar with a tight-fitting lid, and pour in heavy cream or whipping cream, no more than one third full. Shake the jar until you can feel or hear that a solid mass has formed. If the cream becomes too thick to shake easily, add a bit of water. Pour off the liquid and use a spatula or spoon to collect the butter. This soft whipped butter can be served on its own, or mixed with herbs or spices. You can also add flavor by pouring a small measure of flavored liquor in with the cream.
Regional variations 
In Australia both salted and unsalted butter are readily available, and no preservatives, additives or colorings are allowed in standard butter unless they are labeled as a 'spread'.
European butter typically contains 82% to 84% butterfat and comes in salted and unsalted varieties. The greater portion of butterfat makes European butter taste richer than American butter.
United States 
Butter sold in United States markets is typically 80% to 82% butterfat and salted, unless marked otherwise. Flavorings, colorings, and preservatives may also be added. European style butter, at 82% or greater fat content, is referred to as "dry butter", and is available in specialty shops. Salted butter is generally sold in sticks wrapped in wax paper, while unsalted butter is sometimes wrapped in aluminum foil.
South Asia 
Most often used as ghee, or clarified butter.