What is the difference between baking soda and baking powder?

Both are leaveners that make baked goods rise by producing carbon dioxide bubbles, but they are chemically different tools. Baking soda is pure sodium bicarbonate, a base, and it only reacts and releases gas when it meets an acid, buttermilk, yogurt, lemon juice, brown sugar, cocoa, vinegar, plus moisture and heat. That is why recipes using baking soda always include an acidic ingredient, and why the reaction starts fast, meaning soda-leavened batters should go in the oven promptly.

Baking powder is baking soda with a powdered acid, usually cream of tartar, already mixed in, plus a starch to keep it dry. Because it carries its own acid, it activates with just moisture and heat and needs no acidic ingredient in the recipe, which is why it is used in things like plain cakes and biscuits that lack a natural acid. Most modern baking powder is double-acting, releasing some gas when wet and more when heated, giving a little timing forgiveness. They are not swappable one-for-one: baking powder is much weaker by volume, so substituting requires roughly tripling the amount and adjusting for the missing acid, and swapping incorrectly leaves food either flat and dense or metallic-tasting and over-risen. Many recipes use both, soda to react with an acid and brown, powder for reliable lift.