Baking powder is used to expand and add volume baked goods, bread, and cakes. it is a very common ingredient found in a majority of baked goods.
Baking powdered is made of sodium bicarbonate which is typically paired with an acid. Some baking powdered have cornstarch as a filler. Baking powder is considered a leveling agent when used in baking.
A lot of questions are asked about using baking soda instead of baking powder. The difference is that baking soda does not have an acid component it just as sodium bicarbonate. Because of this, it does not have the same leavening properties as baking powder.