What is a Subwoofer Voice Coil?
Inside a circular groove that runs most of the depth of the subwoofer magnet lies the subwoofer voice coil. When the audio signal is fed through this coil, the electrical field interacts with the magnetic field, causing the voice coil and the cone it is attached to, to rise and fall. It is this rising and falling that produces the sound you hear coming from your speakers. Subwoofer voice coils are much bigger and heavier duty than those in regular speakers because they have to handle more power and lower frequencies. A good picture to give you a better idea of what this design looks like can be found here.
For more info, see How to Build the Best Car Speaker System