Baking powder is such a ubiquitous baking ingredient that you may be surprised to learn that it is actually a relatively recent invention in the annals of cooking; the first baking powder was introduced to the market in 1843. Like its close relative baking soda, baking powder acts as a chemical leavener, bubbling when it is moistened and producing carbon dioxide, which leavens foods like breads, cakes, cookies, and so forth. However, the powder is much weaker than baking soda, and it usually acts in two stages, which is why many recipes call for baking powder rather than baking soda, to ensure that the finished product stays light and fluffy.
This leavener is made by blending baking soda (sodium bicarbonate) with a starch to fend off moisture and an acidic compound such as cream of tartar. The earliest evidence of some sort of baking leavener comes in the form of potash (potassium carbonate), an alkaline substance which was derived from pot ashes and used in cooking as early as the 1760s. Bakers realized that the addition of potash and other carbonates could drastically cut down on kneading time for bread. By the late 1700s, sodium bicarbonate had been discovered, and bakers had realized how useful it could be.
Pure baking soda can be a bit unstable in recipes, and difficult to control. In some cases, it can cause a recipe to froth up beautifully when moist, and then collapse when dry. As a result, people began adding other substances to their baking soda to make it easier to control, especially in big recipes. In 1843, Alfred Bird, a British inventor, formulated the first substance we would recognize as baking powder and began selling it, and other companies quickly caught on to the trend.
Most baking powder is double acting, which means that it releases carbon dioxide when moistened and again when exposed to heat, unlike baking soda, which is more reactive. As a leavener, baking powder is about one fourth as strong as baking soda, creating a reaction which is easier to control. This is why it is important not to confuse the two, and if you need to make substitutions, do so with care.
When a recipe calls for baking powder and all you have is baking soda, you need an acid such as cream of tartar to balance out the reaction of the baking soda. Use two parts cream of tartar to every one part baking soda to create the required amount of leavening. If you only have baking powder and you need baking soda, you can multiply the measurement by four, but be prepared for a weird flavor!