Subwoofers are speakers that deliver the lower frequencies, specifically 20-200 Hz, that a traditional two-channel or surround setup can't reproduce on its own. These low frequencies come from instruments such as the kick drum, bass guitar, and pipe organ, as well as movie sound effects like explosions.
A subwoofer (or sub) is a loudspeaker designed to reproduce low-pitched audio frequencies known as bass and sub-bass, lower in frequency than those which can be (optimally) generated by a woofer. Subwoofers are never used alone, as they are intended to augment the low-frequency range of loudspeakers that cover the higher frequency bands. While the term "subwoofer" technically only refers to the speaker driver, in common parlance, the term often refers to a subwoofer driver mounted in a speaker enclosure (cabinet), often with a built-in amplifier.