What is the difference between a single acting baking powder and a double acting baking powder?

Not completely. Double acting baking powder exhibits a chemical reaction in two phases; the first when a liquid is added to the powder (such as when you add milk to turn a dry cake mix into a batter), the second when it is heated (i.e cooked in the oven). This makes it ideally suited to batters which you want to make in advance, but still need to rise.

Regular baking powder only reacts when the liquid is added, meaning that it's not so good if you need to make a batter in advance. (By the time you get around to cooking it, the batter will not rise effectively).

However unless you need to make a batter in advance of cooking it, both types of baking powder are essentially interchangeable.