I have always wondered the difference.
Both, when mixed with salt cause your baking to rise.
BUENO .....that we understand.
But sometimes recipes call for each of the "baking" products as well as salt
WHAT GIVES (what era is that even from?)
In the case of baking powder or baking soda, they act rising agents via a chemical reactions
that releases carbon dioxide gas.
These ingredients are used in QUICK BREADS,
since other ingredient such as yeasts are used for the same purpose but in a longer baking process.
I use then in my muffins, scones & cookies.
BAKING SODA is an alkaline, and when you mix in something acidic, like vinegar, it will release gas. It will work in recipes that include acidic ingredients like buttermilk, sour cream, lemon juice, yogurt and so on. Molasses is also acidic, as well as honey. Baking Soda is activated when moistened.
BAKING POWDER, is nothing more than baking soda with some sort of acidic compound already included. This can be considered MORE POWERFUL since it is activated not only by the salt & to expand, but ALSO THE HEAT.
The reason why both soda and powder might be used in a recipe is because you might have enough soda to neutralize the acid, but not actually enough to lift the batter. In these cases, a little baking powder will give the extra lift needed to make the recipe perfect. You could use baking powder alone, but then your finished baked treat might taste too acidic. Meaning, depending of the amount of the other ingredients you might need to add some extra FLUFF!
IN OTHER WORDS......FOLLOW THE RECIPE IF YOU WANT THE END RESULT!