Arbitrage betting is a type of financial activity that turns in its favor, the price difference between two markets. In the case of arbitrage betting - "price" is the odds, and the markets are two or more competing bookmakers.
Arbitrage betting is when two different bookies give high enough odds for the opposite outcomes that allows you to bet on all possible outcomes and stay in profit at any of them.
Arbitrage situations happen regularly, hundreds of times every day. The person in charge of arbitrage bettings, called "arber". Arbers's profit is usually 5-10% of turnover. Those wishing to learn more will find a tutorial on bookmaker forks "Rating of Bookmakers".