A Subwoofer is a type of speaker that produces low frequencies at high volumes. People mostly use subwoofers in houses for home theater systems like in 5.1 surround sound systems and the it would also be used in cars, those are the most popular ways of where subs are used .
A subwoofer can be made of plastic and sometimes a sort of hardened paper .The skeleton of the sub would be made of metal and then there is the magnet and the coils what produces the power of the speaker.
The coils are like a electric magnet that when power is transmitted on the coils the coils act like a opposite magnetic pole of the magnet and that is how the coil then pushes away from the magnet .You also get dual coil subs and are also bit more expensive.Stick around and i will explain how that works.
Single coils tend to blow more easily ,dual coil usually have more power due to the fact that it has double coil and therefor have more wattage to work with.A subwoofer is more for people who love bass.I bought a 7000w sub the other day and just loving the sound so far.