Baking powder is a dry chemical leavening agent, a mixture of a carbonate or bicarbonate and a weak acid. The base and acid are prevented from reacting prematurely by the inclusion of a buffer such as cornstarch. Baking powder is used to increase the volume and lighten the texture of baked goods. (extracted from Wikipedia)

In breadmaking, baking powder is not very often used, at least not with regular breads. The ones that use baking powder are called 'quick breads', which share a lot of similarities between cakes, but are usually baked in a loaf pan. Banana bread is a classic example of a quick bread.