Baking powder is a leavening agent that is 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 is used in batters where there is no acid present, such as baked goods like cookies, cakes pastries, quick breads, pies, etc.
It makes these types of foods 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, the filler cornstarch is added to absorb the moisture and prevent premature activity.
Today, most baking powders are double acting, which means they react twice; the powder contains one acid that dissolves when it comes into contact with water and another acid that does not dissolve until it reaches a higher temperature in an oven. Single-acting baking powder is mainly used by manufacturers and is not readily available.