A subwoofer is a speaker specifically designed for the reproduction of very low frequencies (ie. bass).
Low frequencies need to be produced by large-diameter speaker cones which is why subwoofers are usually so large. It takes a lot of current which is why subwoofers usually have their own amplifiers. You can tell when a subwoofer is in operation because the building will be gently vibrating.
EUTC has in the past hired subwoofers for shows, usually when it is desirable that the audience by violently vibrated for dramatic effect.
As of 2008-07-18, Bedlam actually owns a subwoofer (apparently we lost it, then we found it again). It is connected to a hideously oversized mixing amplifier that is kicking around beneath the sound desk. It distorts easily but with a little care you can get a pretty good rumble out of it.