Baking powder in its most basic form is baking soda mixed with a dry acid. There are two main types of baking powder: Single acting Baking Powder and Double Acting Baking Powder.
To make a simple single acting baking powder mix 2 tsp cream of tartar with 1 tsp of baking soda. If using in baking add 1 tsp of salt per 1 cup of flour.
Single Acting Baking Powder
Single acting baking powder is classified by the type of acid it is mixed with. S.A. baking powder react quickly when mixed so it is important (if being used in cooking) to bake the batter quickly after adding the baking powder.
Double Acting Baking Powder
Double acting baking powder is by far the most commonly sold in stores. The acid(s) in this baking powder do not react until dissolved in a liquid (so your baking powder doesn't start reacting while being stored. Once the baking powder has been added to a batter the baking soda and the dry acid can dissolve and begin reacting.