Baking soda is alkaline, and when mixed with acidic ingredients, it reacts and releases bubbles of carbon dioxide. These bubbles, when trapped inside batter, help baked goods rise. Baking powder contains baking soda, along with acidic salts that react with the soda when they get wet or heated. Recipes that call for both baking powder and baking soda are probably using the baking soda to offset extra acidity in the batter (from ingredients like buttermilk or molasses) and to weaken the proteins in the flour. Omitting the baking soda from these recipes may alter the color or flavor of whatever you're baking, and make it less tender.
Shelf life: 2 years