There are a few key differences between bobbins and spools that you should be aware of before using either one. Perhaps the most obvious difference is size; bobbins are much smaller than spools. This is because they're designed to hold less thread than a spool. Bobbins are also typically made of plastic, while spools can be made of either plastic or wood.
Another important difference is that bobbins typically have a flanged edge, which helps the thread stay in place while you're sewing. Spools, on the other hand, don't usually have this feature. This means that it's easier for the thread to come loose from a spool than from a bobbin. Finally, bobbins typically have a hole in the center so that they can be attached to a sewing machine, while most spools do not have this feature.