So, what's the difference?
The simple answer: Baking soda needs an acid. Baking powder has an acid.
The long answer:
Since both baking soda and baking powder act as leaveners, they are used in baking to make doughs rise. Because their chemical reactions occur quickly, they are most often used in "quick bread" recipes (i.e. muffins, biscuits, scones, even pancakes).
When using baking soda in a recipe, remember that it needs an acid in order to activate a chemical reaction - an ingredient such as buttermilk, sour cream, lemon juice, vinegar, yogurt, etc. Think back to your science class days, did you ever build a baking soda and vinegar volcano? If so, you were witnessing the reaction of acid + sodium bicarbonate (i.e. baking soda) = release of carbon dioxide; this release of gas is exactly what you want to happen in your dough, it means it will rise! Failure to include an acid will result in the baking soda not being activated which equals flat biscuits. This also means if you try to sub baking soda for baking powder in a recipe where no acidic ingredient is present, you'll have a flop result.
Baking powder, on the other hand, already contains the acid; it's actually baking soda mixed with an acidic compound. This means that you don't have to include any additional acidic ingredient - the soda and the acidic compound that make up the baking powder won't start their chemical reaction until you moisten them. If your baking powder is labeled "double acting," this means that the chemical reaction will be further activated by the heat of the oven/griddle/heat source.
Can you substitute one for the other? In theory, yes, but all too often the results will not be pretty. If you substitute baking powder for baking soda on a 1:1 basis, you won't get enough rise in your dough; baking powder is approximately 1/3 soda and 2/3 other ingredients, so you'll only be adding a third of the soda you need. Alternatively, if you were to sub baking powder for baking soda on a 3:1 basis, you'd risk having a bitter taste and too much leavening, meaning the dough would quickly rise and then fall before the bubbles had a chance to bake, translating to a flat, tough biscuit or scone. If you try to substitute baking soda for baking powder, you will likely be missing an acid, which means the soda won't activate and you'll have flat baked goods. Additionally, because soda has a metallic taste that is typically neutralized by the acid, you'll also be risking ending up with a metallic tasting muffin. It's much easier just to keep a box of baking soda and a can of baking powder on hand!