They're both leaveners to lift the cake batter.
They can be used singly or together depending on the other ingredients in the recipe and somewhat by how it will be worked and cooked. Buttermilk or vinegar are commonly included in recipes that use baking powder or look for some other acidic ingredient, such as cream of tartar.
Baking Soda is a base. It's coupled with acidic ingredients as acids and bases react to release carbon dioxide. It reacts as soon as it gets wet.
Baking powder is a base, often baking soda, and some powdered acids. Usually two types: one that reacts to moisture, and one that is heat activated. These two separate reactions are why it's called double acting baking powder. This is more commonly used where some extra lift is desired during baking and/or where there aren't acidic enough ingredients.