Baking soda is also known as sodium bicarbonate. It is a more powerful leavening agent than baking powder. Baking powder is sodium bicarbonate + cream of tartar + starch.
I have to disagree that Baking Soda is a more powerful leavening agent than Baking Powder. Baking Soda begins it's leavening action almost immediately and is used greatly in many types of cookies that too much leavening would ruin. Baking Soda is a "single" action leavening agent while Baking Powder is generally a "Triple" Action leavening agent.
Baking Powder works in 3 stages (thus the term Triple Acting). The Sodium Bicarbonate is activated almost immediately in the recipe and begins the leavening of the product when liquid is added to the mixture. The Cream of Tartar that is in Baking Powder is activated by some type of "acidic" ingredient, this could be citrus, buttermilk, etc. The starch is then activated by "heat", once placed in the oven for baking and a certain temperature is reached, this activates the remaining leavening agent, creating the "third" stage and leavening the product to the maximum amount.