<font color='#000000'>cross'-o-ver : transferring from one state of consciousness to another...
OK, just kidding!
A crossover is either an internal or external system design to separate frequencies and send them to different output devices (i.e. "loudspeakers"
These can be active or passive... This is generally done to a) prevent damage, b) avoid distortion, and c) balance the system for optimum performance.
Lets build a simple passive 'low-pass' crossover. Take about 50' of copper wire, wind it around a piece of curved pipe and send a signal through it. Voila! The output will have lost much of the high-frequency information it started with... you've just created your first cheap, sloppy, low-pass crossover. A crossover is not a brick wall, however, it blocks frequencies gradually, resulting in a "slope" of dB loss over frequency.
An example of a passive high-pass crossover would be running the same signal through a capacitor (I'll omit values for simplification). This would allow the high frequencies through and 'block' much of the low-frequency information.
Active crossovers work while the signal is at low voltage and require external power to work. Active crossovers can also generally be adjusted, while passive crossovers cannot (without replacing components.) An EQ is a form of an active crossover.
Most Audiophile crossovers are going to be composed of several high quality components which separate the audio signal into differning bands (frequency ranges) which are then sent to the appropriate output device. High frequencies go to tweeters, low frequencies go to the woofers, etc...
This is an oversimplified explananation, but should give you the basic idea behind the term. There are likely passive crossovers inside every bookshelf speaker and active crossovers within every receiver.
[Edit: darn spelling]</font>
