Baking soda acts immediately creating CO2 gas the moment it mixes with an acid (for example buttermilk, milk, sour cream, vinegar).
Do you remember the baking powder propelled bath tub toy submarines?
Try mixing a table spoon of baking soda with just a few drops of vinegar to see the kind of action we're talking about. But once the reaction is over, that's it, no more.
Baking powder creates some CO2 gas when acted upon by moisture, and not necessarily and acid. If it's double acting, it also creates more gas when it's heated above a certain temperature during the baking process, giving the baked item even more lift.
This is important when the internal structure of some baked items (for example cake batter) needs to dry out a little in the oven before it can successfully hold the gas bubbles inside without letting them just bubble away, leaving you with a heavy blob of dried out batter. Some bakery items just need time before gas bubbles can properly do their "thing", so baking powder gives that extra delay of time that is needed.
Bottom line: Baking soda and baking powder are NOT interchangeable!
"Food is our common ground, a universal experience." - James Beard