Hache,
When I signed up over a year ago, there was an interactive tutorial that shows you step by step how it all works. Others can give you much better lessons than I can since I have spent less than 300 hours trading at the site.
The way it basically works at tradesports is this:
you have two teams, and on a football game you have the usual spread (moneylines are offered too). On the spread you also have a price attached- similar to Pinnacle's changing vig. You actually buy shares of a team at an actual price, and that team must win by the spread. A winning team's price closes at 100 (when the clock hits 0:00) and the loser's price closes at 0. So if you bought a team to win by 2.5 at 54 your "shares" will hit 100 if they win by 3 or more, and your share price will hit 0 if they dont (total loss).
But the beauty of interactives/exchanges is that you can buy or sell DURING THE GAME. The price changes, of course, after certain things happen (scores, injuries, turnovers etc.) and the price is dictated by the public's buying and selling of each teams stock. One way to utilize the exchanges/interactives is say you handicapped a game based on a running back/offensive line amassing 150 yards rushing in consecutive games going against a team with a weak D-Line. In the first quarter it becomes obvious that the defensive unit has done their homework and employs a defense which confuses the running offense. Instead of being stuck with that wager, you can sell for a profit or loss at any point during the game which is much better than being stuck with it for the whole game IF you know what you are doing. When I say "the price changes" I mean the price at which someone will buy the shares that you are trying to sell. You can pretty much offer your stock for sale at any price you want, but people will buy at the best available price, and this can change VERY quickly. There are many complex options available similar to the stock market, such as setting margins etc, but I'll let other more qualified people get into that. The vig is also pretty small which is a plus.