Both baking soda and baking powder are rising cooking agents, i.e., substances which assist the rising process in cooking.

Their difference lies in the fact that baking soda is pure sodium bicarbonate while baking powder contains sodium bicarbonate (i.e., baking soda) as well as an acidic agent and a moisture absorption agent (usually starch).

For this reason, some recipes require baking powder (usually cakes) while some others require baking soda (usually biscuits), depending on the other ingredients used in the recipe. If there are other acidic ingredients in the recipe, we usually use baking soda; if not, we use baking powder.