What is the difference between baking powder and baking soda?

Baking soda is pure sodium bicarbonate (NaHCO3), while baking powder blends sodium bicarbonate with a dry acid (commonly tartaric acid or cream of tartar) plus a moisture-absorbing filler like starch. Baking soda needs an external acidic ingredient (e.g., buttermilk, lemon juice) to release CO2 for leavening; baking powder already contains the acid, so it produces CO2 when moistened and heated. Baking powder is typically 'double-acting,' giving off some CO2 at room temperature and more during baking, whereas baking soda reacts only once, immediately after it meets an acid and liquid.