One stick of butter equals about 1/2 cup. Some recipes will give butter in cups, but will also include measurements for the sticks. Some websites will even convert certain measurements to make it easier to follow the recipe.
U.S. butter is typically sold in packages of 1/2-pound or 1-pound sticks, although you can get tubs of butter meant more for spreading on bread than for baking. The butter sticks are individually wrapped and weigh about 1/4 pound, or 1/2 cup. The wrapping over the butter includes various measurement breakdowns so it is easier to take the exact amount needed for the recipe.