Kalid at BetterExplained has an excellent article on the various flavors of "averages" and their uses.
He runs a very informative and interesting blog that explains a variety of concepts in a the-light-bulb-over-my-head-is-finally-glowing way. I recommend it for just about anyone who wants some insight into the world around them.
BTW, to better appreciate the quality of BetterExplained's take on statistics, just compare it to my attempt:
(Please give me bonus points for the catchy title, though.)