Baking powder is basically used as a leavening agent during baking. It is a mixture of baking soda (particularly Sodium bicarbonate) and a dry acid. It also contains corn starch that acts as a buffer.
Baking powder is used while preparing the dough of the bread or the batter for the cake, basically during the initial steps of baking. It increases the volume of the dough or batter and leavens the texture of the same. How does this happen?
Well, baking powder contains a carbonate (base) and a dry acid which recats to release carbon dioxide. The bubbles formed by this gas increased the surface area which in turn increases the volume of the dough or batter. Thus baking powder increases the volume of baking products.