If you look at the odds given by bookies in a typical sports event you will notice the sum of the probabilities will add up to an amount greater than one.

Here's an example using a hypothetical horse race:

Contestant    Odds    Probability                                     
Horse-1       2-1     0.3333                                          
Horse-2       4-1     0.2000                                          
Horse-3       4.5-1   0.1818                                          
Horse-4       5-1     0.1667                                          
Horse-5       6-1     0.1429                                         
Horse-7       7-1     0.1250                                         
Horse-8       8-1     0.1111                                         
Horse-9       9-1     0.1000                                         
Overround:            1.3608 (sum of probabilities)

1. The overround in this case is > 1. If you think back to any statistics course you may have taken you recall that the sum of all probabilities must equal exactly 1.
2. The amount in excess of 1 is the bookmaker's profit. The larger the overround the greater the profit for the bookie.

You'll generally be on the losing end of any given bet, but only considering events with small overrounds minimizes the house's advantage.