What's baking?

 
Sodium bicarbonate (baking soda) tends to produce carbon dioxide (CO2), when mixed with an acidic substance.  This accounts for dough or batter bubbling when sodium bicarbonate in baking soda is added to ingredients like sugar, milk, or shortening. However, baking soda alone tends to become unstable at higher temperatures. So for example, one might see baking soda in recipes for pancakes, or cookies, because these food items don’t require long baking times.
Recipes that generally don’t take baking soda, but instead require baking powder (baking soda + cream of tartar) generally have longer baking times. 
This is because the cream of tartar in baking powder acts as a second leavening agent, and takes over when the baking soda, or sodium bicarbonate, is rendered inactive by long exposure to heat (Cream of Tartar is potassium hydrogen tartrate).

Thus one often sees baking powder required in recipes like muffins, cakes, and non-yeast breads. It is also popular in recipes like biscuits, which have to cook at a high heat.
It is possible to substitute baking powder for baking soda, though one may need to add a larger amount of powder to get the same results. However the reverse is not true. Substituting baking soda for baking powder will not work in recipes that require high heats or long cooking times. As well, baking soda has a somewhat bitter taste, which can be hard to mask.
If one is in a bind and the store is closed, baking soda mixed with cream of tartar is essentially “do it yourself” baking powder. Mix two parts of cream of tartar to one part of baking soda to make baking powder. This will also dull the taste of the baking soda, so that it won’t give off a bitter taste.
From WiseGeek.com http://www.wisegeek.com
Me personally I try to bake without any chemicals,  but if you use baking powder, try to find one without aluminium.