It is that you buy or sell shares for the price you think is right (if you can get that price). Just like shares of a stock.

Or more exactly, it is like a commodity futures market, where you buy the right to buy the commodity for a certain price, at a certain time. If, when the time comes, the actual price of the commodity is more than you can buy it for, then you make a profit. If the price of the commodity is less, then your shares become worthless. Did you see the film 'Trading Places' with Eddie Murphy, Dan Ackroyd, Jamie Lee Curtis (surprising topless scene!)? That film goes to a great deal of trouble to explain commodity futures.

Now there is a difference between commodities futures and Intrade, in that with Intrade, there is no crop of frozen oj or pork bellies with real tangible worth. Not sure how it is handled in Intrade, but I suspect that when the term is up, the winners divide up everything that was invested in the shares somehow? Not sure how this works.

But prior to the end of the term, it is just buying low and selling high.