A subwoofer is essentially a speaker that is designed to play the lowest bass frequencies replicated by your stereo system. Regular speakers reproduce a wide frequency range but often lack the deepest bass frequencies. A subwoofer uses a larger woofer as well as a built-in amplifier that powers the signal to provide deeper, cleaner bass. Most subwoofers are powered, meaning they require their own individual power supply from an electrical outlet to power a built-in amplifier. Passive subwoofers do not require their own dedicated power supply and use power from the receiver's amplifier.
Receiver to Subwoofer
Subwoofers are connected to an audio receiver or preamplifier. The receiver sends low frequency (usually 20 Hz to 100 Hz) signals, refered to on soundtracks as low frequency effects (LFE), via electrical current to the subwoofer. The subwoofer amplifies the current and converts it to sound through the use of a magnetic coil that causes vibration of the driver's cone. This vibration results in low frequency sound waves being sent out through the air.
Low frequency signals are not directional the way higher frequencies are. This means that the listener has more flexibility in the placement of the subwoofer, unlike the left and right speakers which are very sensitive to placement and generally need to be directed at the listening position. A subwoofer can be moved around based upon the layout of the room. Experimentation with different positioning can also help to ascertain the best location for sound quality.