If I can add to what Turnbull said.

When you bet with a bookie it costs $11 to bet $10.

The one dollar is the "vig" or price of doing business.

So, if Charlie the bookie takes 10 $10 bets he really takes $110.00

If all the bets are for one team to win, say for the Reds to beat the Yankees. Charlie will then get in touch with someone who has 5 bets for the other team. So Charlie has 5 for one and 5 for the other. The wins cancell the losses and he makes $10. Multiply that by thousands, and factor in point spread and all that and you can see it adds up to some serious $$.

Hope this helps.