Baking powder was invented to make cakes rise without using eggs, i.e. a leavening agent. It results in a fluffier lighter beast, but with somewhat less staying power compared to old-fashioned cakes.
Baking powder comprises bicarbonate of soda (an alkali) and an acid, traditionally cream of tartar plus a filler like cornflour to keep the alkali and acid separated before you need them mixed. Baking powder is either single action or double action. Single action means that it starts reacting immediately the two components are mixed during the preparation process, whilst double action really kicks in when the cake mix is also heated. These days most baking powders are double action and comprise bicarbonate of soda plus acid chemicals such as Sodium Acid Pyrophospate (slow acting) and/or Monocalcium Phosphate (fast acting).
Tip: look for baking powders that contain no aluminium-based compounds as this reduces any metallic tastes to your cake.
To use: follow your recipe or (as a guide) add 1 tsp (level) of baking powder to 125g of plain flour.
To make your own self-raising flour: add 1 tsp (level) of baking powder to 125g of plain flour. Mix really well.
Dowell, P., Bailey, A. (1980) The Book of Ingredients, Dorling Kindersley. ISBN 0718119150.