What happens when you use baking powder instead of baking soda?

Substituting baking soda for baking powder helps the food to rise more, but also affects the flavor if you use too much.

Baking soda begins to release bubbles as soon as it gets wet, so you lose rising power if you delay getting the batter into the oven. Baking powder (which may be called "double acting") has some ingredients that don't release bubbles until they are heated, so it is a little more forgiving.

Baking soda helps reduce the acid in sour ingredients such as buttermilk, sour cream, yogurt, and molasses; that is why some baked goods have both.

Usual amounts are 1/4 teaspoon baking soda per cup of flour or 1/2 teaspoon baking powder per cup of flour.

Here is a lengthy article comparing the two:

http://www.ellenskitchen.com/pantry/bsbp.html