I just heard about them recently. It basically lets you bet money on the likelihood of world events occurring in an open market.
Here's a short FAQ for one:
http://www.intrade.com/v4/misc/howItWorks/theBasics.jspEach event fluctuates between having a value of $0 and $10 per share. Once the event occurs or does not occur, the shares will go to $10 or $0, respectively. The market decides the price of the shares in between, and this done based on the perceived likelihood of the event occurring. Important to remember that: yes = $10, no = $0.
To illustrate: there's one on there about Mitt Romney becoming the Republican nominee. Right now, shares are going for about $7, so he has about a 70% chance of winning according to these random people. If I bought 100 shares at $7 for a total of $700 and Romney ends up winning, then the price would go up to $10 per share. This means I would win the difference between $7 purchase price and the $10 final price, and there's no transaction costs or anything. So $3 per share x 100 shares = $300 profit. That money comes from all of the people who bet on the other side of the issue, who lose all of their money.
Seems like a gambling addicts wet dream. I wanted to play around a little, but 1) not sure of legality in US, 2) they don't take US credit cards, 3) there's a $5 a month charge for account usage.
Womp womp