Baking soda is a "salt" – the chemical name is sodium bicarbonate. As such, it has a lot of sodium, about 1200 mg of sodium in a teaspoon of baking soda.
When baking soda is mixed with an acid (like buttermilk or yogurt) the combination forms carbon dioxide gas, which is what makes batters and dough rise. The reaction takes place immediately so the baking soda should be added to the dry ingredients first and then the liquid ingredients mixed in.
Baking powder is made up of baking soda and cream of tartar. Because baking soda needs an acid to create the carbon dioxide that helps baked goods rise, the addition of cream of tartar makes baking powder an easy to use leavening agent. Single acting baking powder means that the baking powder reacts immediately when liquid is added. Currently, single acting baking powder is not readily available in the U.S. Double acting baking powder releases carbon dioxide in two stages. The first stage occurs after wet ingredients are added and has a second release when the when the mixture is heated.