Some just raise the 'Book up a bit to give more air circulation underneath, but the good ones plug into the USB port for power and have little fans that actively circulate air under the computer.
And while I've never used one myself, I've heard good things about them. I was actually about to buy one for my 17" MBP, not because it's frighteningly hot, really (no worse than the last Dell laptop I used, anyway), but just to make it more comfortable to put on my lap or a soft surface without worrying about heat buildup.