With a home stereo, the power output from the one and only amplifier is divided (by something called a crossover) inside the speaker box so that the low frequencies go to the larger speaker and the high frequencies go to the smaller one (the infamous woofer and tweeter respectively). In a very fancy stereo, it may be split across 3 (or even more) speakers.

In a large PA system, the signal is split before the amplifiers (also by a crossover, but this one is an active crossover) and each speaker has its own amplifier. This obviously requires more amplifiers but a major system will have up to a hundred or so anyway. The advantage is that you only need one crossover (actually, two - one for left & right) and it works at the much lower amplifier input voltage so it doesn't absorb any of the amplifier's power.

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Note added at 21 mins (2010-03-17 12:20:18 GMT)
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This is actually a fairly mild technical rider - the requirements seem reasonable and reasonably flexible. You should see some of the one's I've had to deal with :)

Wait until you get to the bit about the brown M&Ms and no pork products to be present on site!