Differences:
Bi-carbonate of soda is a pure leavening agent. It needs to be mixed with moisture and an acidic ingredient for the necessary chemical reaction to take place to make food rise. Because it needs an acid to create the rising quality, it is often used in recipes where there is already an acidic ingredient present, such as lemon juice, chocolate, buttermilk or honey.
Baking powder, which contains bicarbonate of soda, comes pre-mixed with the acidic ingredient for you – so all you need to add is the moisture. The acidic ingredient most often used in baking powder is cream of tartar. You can make your own baking powder: simply mix two parts cream of tartar with one part bicarbonate of soda. Baking powder has a neutral taste and is often used in recipes that have other neutral-tasting ingredients, such as milk.