Baking soda will create leavening on its own when it is heated (try pouring boiling water over baking soda in a sink to help unclog a drain, it will bubble up!), but unless it is balanced with an acidic ingredient, the resulting taste may be metallic. Baking powder is a mixture of baking soda and a dry acid, such as cream of tartar, and perhaps some corn starch to help keep the two separate and dry. Most baking powders on the market are “double acting”, meaning that some leavening occurs the minute the baking powder gets wet, and the rest of the leavening occurs when it is heated.
Baking powder is sodium bicarbonate and baking soda is sodium carbonate.