Baking powder is a dry chemical leavening agent made of baking soda (sodium bicarbonate) combined with one or more powdered acids, such as cream of tartar or sodium aluminum sulfate, plus a starch to keep it stable in storage. Unlike plain baking soda, it needs no separate acidic ingredient in the recipe - it reacts with moisture and heat on its own to release carbon dioxide, which is why it works in otherwise-neutral batters like plain cakes or biscuits.
Most baking powder sold today is 'double-acting': it releases some gas on contact with liquid and more when heated in the oven, giving batters two chances to rise. It's used in cakes, quick breads, biscuits, pancakes, and waffles, and is generally flavor-neutral.