Butter is made by churning cream to the point where the fat separates from the water and milk solids. Some of the water and milk solids are left in, usually 20%. For most recipes you will want to use "unsalted" or "sweet" butter which has had no salt added to it. Salt is added to butter to preserve it giving it a longer shelf life. For this reason many chefs prefer not to use it because it may be old.
