Both baking soda and baking powder are leaveners, which means they make cakes and muffins rise. So what's the difference?
Baking soda is pure sodium bicarbonate. Not only will baking soda fluff up your muffins, it can also put out a grease fire, clean your teeth and deodorize your carpet. When you cook with baking soda, you need to balance it out with an acid like lemon juice or buttermilk. Otherwise, your muffins may have a bitter, soapy flavor.
Baking powder, which usually comes in a can, is a combination of baking soda and a few other ingredients, most notably cream of tartar, a dry acid. Out of baking powder? Make your own by mixing one part baking soda with two parts cream of tartar.
Here in Germany I have been told that one can find cream of tartar, or weinsaure at the Apotheke, although I have not tried it yet. Baking soda and baking powder are best found, here in Berlin, at my local Lebanese market, where I can get it in large packets, as opposed to the 1 T packets sold in the German supermarkets. Warning: baking powder here is not double reactive. For me, that means that the mix should go in the oven as soon as possible after hitting fluid.