Baking powder is a raising agent that is commonly used in cake-making. It is made from an alkali, bicarbonate of soda, and an acid, cream of tartar, plus a filler like cornflour or rice flour which absorbs moisture.
The powder is activated when liquid is added, producing carbon dioxide and forming bubbles that cause the mixture to expand. For this reason, it is important to get your cake mixture into the oven quickly once the 'wet' ingredients have been added to the 'dry' ingredients.
Self-raising flour is made from plain flour combined with a small amount of baking powder.