A bookmaker (who often works from a betting shop, though increasingly over the Internet or by telephone) will open a book of bets. This means that they will come up with a range of bets and an opening betting market for a particular event. As the betting proceeds, they will change the odds so that they can maintain a profit.
All the odds in an event multiplied by all the bets laid at those odds will never be greater than the money a bookmaker has taken in bets.
See also Bookmaker.