Bi-Carb Soda, also referred to as Baking Soda, releases carbon dioxide when in contact with acid.  It is commonly used as a raising agent in baking.

Baking Powder is a mixture of Bi-Carb Soda, starch and at least one slightly acidic compound such as cream of tartar.  It works as a leavening agent in baking by releasing carbon dioxide when mixed with a liquid, such as milk or water.

Both are considered to be raising agents, however are used under different conditions depending on the mix of other ingredients within the recipe.