Same thing
http://en.wikipedia.org/wiki/Baking_soda
Applications
[edit] Cooking
Main article: leavening agent
Sodium bicarbonate is primarily used in cooking (baking) where it reacts with other components to release carbon dioxide, that helps dough "rise". The acidic compounds that induce this reaction include phosphates, cream of tartar, lemon juice, yogurt, buttermilk, cocoa, vinegar, etc. Hence, sodium bicarbonate can be substituted for baking powder provided sufficient acid reagent is also added to the recipe.[3] Many forms of baking powder contain sodium bicarbonate combined with one or more acidic phosphates or cream of tartar.
Baking Powder is however slightly different due to acidity levels
http://en.wikipedia.org/wiki/Baking_powder
Substituting in recipes
Baking powder is generally just baking soda mixed with an acid, and a number of kitchen acids may be mixed with baking soda to simulate commercial blends of baking powder. Vinegar (dilute ethanoic acid), especially white vinegar, is also a common acidifier in baking; for example, many heirloom chocolate cake recipes call for a tablespoon or two of vinegar.[citation needed] Where a recipe already uses buttermilk or yoghurt, baking soda can be used without cream of tartar (or with less). Alternatively, lemon juice can be substituted for some of the liquid in the recipe, to provide the required acidity to activate the baking soda.