Baking Powder is a leavening agent that consists of a combination of baking soda, cream of tartar, and a moisture absorber (like cornstarch). It has the action of yeast, but it acts much more quickly. It's used in batters where there is no acid present, such as many baked goods: cookies, cakes, pastries, pies, quick bread, etc. It makes these types of food products voluminous by allowing gas formation when an acid comes into contact with it and/or when it's heated. Baking powder acts immediately upon addition of water, therefore a filler (usually cornstarch) is added to absorb the moisture and prevent premature activity.