The basic principle of bookmaking is straightforward and quite obvious. A bookmaker takes money in whenever they lay a bet to the customer, and they pay money out every time one of their customers wins a bet. The idea is to take more money in than pay out. The art of bookmaking is in making sure this happens.
Bookmakers can’t control the outcome of sporting events, but they can control how much they stand to win or lose on any particular result. They set the odds for all the bets they lay, which ultimately enables them to make profits.