It's not outsourced computer power, not really. This isn't a setup like f@h where they have a job they need done so they distribute it out to whoever wants to volunteer. From what I gather, when you 'mine' bitcoins, all you are doing is going through the computations to verify past transactions. So, the person who created mining paid himself x bitcoins from one account to another, and then that transaction had to be verified a certain number of times. The people who did the verification were "paid" (in bitcoins) for their work. Well, now you have more transactions to verify, and thus bitcoin mining was born. That's how I understand it. It's actually quite difficult to nail this stuff down (lack of information).
Anyway, so for a while, people did this with their cpu's. It was slow but steady. Then someone ported it to work on gpu's, much faster, that's when everything exploded and people started making lots of bitcoins. However, just like with any currency, you don't want too much of it flooding the market, so they limit the amount of bitcoins by the difficulty and reward factors. The more mining power there is, the more you need to earn bitcoins, thus you fight rapid inflation and try to stabilize the value of your currency. When fpga's and specifically designed ASICs starting showing up, it started to make gpu's irrelevant, just like gpu's made cpu's irrelevant.
It's basically one big economic experiment. If you got in early, you scored. If you're new to the game, there's not much left for you without a large upfront investment and no guarantees. Lots of people tried to make it a legit currency as well by offering other ways to get bitcoins and trying to setup exchanges and such, but with the falling popularity of "free money", all of those things are starting to go away also.
That's about all I know about bitcoins