It is like buying and selling a stock or future contract. There is usually a small spread (ex. Buy 4, Sell 2). That means you can buy them at 4 and sell them at 2 currently. For most markets there is one winner and each winning contract is automatically worth 100 when the event ends.
There are fluctuations throughout the competition and you can in fact sell before it is over. I use WSEX futures for American Idol and if you know what you are doing or are lucky it can be a gold mine.
Here's an example:
You like the San Fran 49ers to win the Super Bowl.
Currently each contract will cost you $3. If you sold right away, you can sell at $2. Thus losing $1 on each contract.
If you hold until the end, each contract will be worth either 0 (if the Niners don't win the Super Bowl) or $100 (if the Niners do win the Super Bowl).
Let's say the Niners start the year 7-1. Likely at that point each contract will be worth significantly more than you started out at. In our example, after Week 8, SF is buy 18 sell 15.
At this point you say it is time to cash in. You can sell what contracts you have at that point for 15.
Each contract you bought at $3 will net you $12, (sell price of $15 minus buy price of $3).
Short selling I will not getting into right now, but if you bet against someone you must put up $100 (the price of a winning contract) and will win whatever the current price is.