Baking powder is a leavening agent composed of baking soda and one or more acidic salts, such as cream of tartar or dicalcium phosphate dihydrate. When the baking powder is combined with liquid, the baking soda reacts with the acids to produce carbon dioxide gas bubbles, which are trapped by the dough around them.
Most baking powders are double-action, which means they have two or more acids in them. The first acid reacts to produce carbon dioxide at room temperature, and the second only reacts once heated, as in the oven.
Baking powder is used in recipes where the pH is 7, or neutral, and pure baking soda would fail to produce bubbles. It is often found in quick breads like pancakes, waffles, and muffins.