Cream of tartar has a multitude of uses in the kitchen. It stabilizes egg whites during meringue making by denaturing the proteins in the egg whites. This allows control over protein coagulation and leads to an increase in volume. It is used as a catalyst in amorphous candy formation (ex: caramel) to convert sucrose into the invert sugars glucose and fructose. This leads to a resistance to crystallization. It is also used in baking powder to activate baking soda.
To create a substitute for baking powder, use ½ tsp of cream of tartar and ¼ tsp of baking soda for every 1 tsp of baking powder desired. It might seem strange that the cream of tartar and baking soda does not add up to 1 tsp, but that is because actual baking powder contains a starch filler which stops the baking soda and cream of tartar from activating before the addition of water.