Why do some cookie recipes call for baking soda and others call for baking powder?

Soda and powder are used as leavening in baked goods.

Baking soda is a "basic" substance -- that is, something with a high pH -- which, if you remember from high school chemistry, will readily produce carbon dioxide when mixed with an acidic substance. Carbon dioxide, a gas, helps products rise. You will see baking soda used in recipes that contain acidic ingredients like yogurt, sour cream, buttermilk, brown sugar, chocolate, applesauce, orange juice, molasses and so on.

Baking powder combines acid and base in one product, and it doesn't require the addition of an acidic ingredient in the recipe to help the product rise.