A crossover is a filtering device that takes a music signal and splits it into separate frequency ranges. This is usually done in order to send them to speakers that are tuned to best reproduce each frequency range. In a typical speaker, there is a tweeter for high frequencies, and a woofer for low frequencies. In some, there is also a midrange speaker to make a smoother transition. In a Bose lifestyle-type system, the tweeters and midrange are in the satellites, and the woofer is in the big box you hide (bass is omnidirectional, meaning your ears can't tell where it comes from.)

There are three types of crossovers: passive, active, and acoustic, and each has their strengths and weaknesses.

A passive crossover is one where the filtering is done by a circuit that is not powered by an external source. If used to only block one part of the signal, such as high or low, they are referred to as a high-pass (lets the treble through), bandpass (lets only the midrange through), or low-pass (lets the bass through) filter. This is often done to protect smaller speakers in simple installations, such as door speakers in a car stereo.

An active crossover is one where the filtering is done by a powered electronic circuit. This is usually used when an amplifier is tasked to every speaker covering a specific frequency range. This ensures that the filtered signal is clean and of sufficent level to operate the amplifier, and also saves power as the amp only drives the selected frequencies. They can also be found in many quality car stereo setups, as a good one will have at least a dedicated subwoofer amp.

An acoustic crossover is one where the filtering is done purely by the construction of the speaker enclosure. A properly designed ported box, for example will naturally only reproduce bass frequencies, "filtering out" the treble. Most non-powered subwoofers use this method. For a description of the various subwoofer enclosure types, go to my subwoofer writeup.