Being curious, I googled it and found this article: Difference Between Baking Soda & Baking Powder
Baking Soda
Baking soda is pure sodium bicarbonate. When baking soda is combined with moisture and an acidic ingredient (e.g., yogurt, chocolate, buttermilk, honey), the resulting chemical reaction produces bubbles of carbon dioxide that expand under oven temperatures, causing baked goods to rise. The reaction begins immediately upon mixing the ingredients, so you need to bake recipes which call for baking soda immediately, or else they will fall flat!
Baking Powder
Baking powder contains sodium bicarbonate, but it includes the acidifying agent already (cream of tartar), and also a drying agent (usually starch). Baking powder is available as single-acting baking powder and as double-acting baking powder. Single-acting powders are activated by moisture, so you must bake recipes which include this product immediately after mixing. Double-acting powders react in two phases and can stand for a while before baking. With double-acting powder, some gas is released at room temperature when the powder is added to dough, but the majority of the gas is released after the temperature of the dough increases in the oven.
Substituting in Recipes
You can substitute baking powder in place of baking soda (you'll need more baking powder and it may affect the taste), but you can't use baking soda when a recipe calls for baking powder. Baking soda by itself lacks the acidity to make a cake rise. However, you can make your own baking powder if you have baking soda and cream of tartar. Simply mix two parts cream of tartar with one part baking soda.
I find this stuff to be fascinating, so if that makes me a nerd, so be it.