Both are leavening agents which help foods rise and be a bit puffier.
BAKING SODA requires the addition of an acid often apple cider vinegar or lemon juice. Other foods that can act as an acid are cocoa, chocolate, honey, and maple syrup.
BAKING POWDER already has the acid mixed in. Recipes usually require 2 to 3 times as much baking powder as baking soda. Additionally, if it is double acting baking powder then it will cause rising when it gets wet, just as baking soda does. However, it will also cause rising when it is heated (which is why it is called double acting…it has two ways of causing rising action.)
To simplify my baking, I usually use baking powder. Some people say it can change the taste of food. I haven’t really noticed that, though a friend says she prefers to bake almond flour bread with baking soda and apple cider vinegar to give a slightly yeasty flavor to her bread. If I am using baking powder in a recipe that calls for baking soda I double the amount called for and cut the amount of salt by more than half. I make sure the baking powder I buy does not have aluminum, gluten, or gmo corn starch.