Arbitrage (or sure bet) is type of betting where the player wins with certainty without depending on the outcome of the game. Usually the player gain only a small percentage of a total bet, but it’s an investment without any risk. Arbitration happens because of the big variety of online bookmakers, each of which has a specific market and customers.Full information about Sure bet rules on Arbitrage betting at WiKipedia
For example to take two football teams: Real Madrid and Barcelona. Some Bookmakers offer bets for Real Madrid and its coefficients are gone down while the coefficients for Barcelona are rising. But the others Bookmakers offers the opposite. This leads to a situation where the bettor can bet on Real Madrid in one bookmaker and Barcelona in another and still have a profit.
As you can see both bookmaker odds vary greatly. So if you bet on Barcelona 3.0 and of Real Madrid 1.6, you will have a sure profit. If we assume that you want to win € 100 of this match, you will achieve it by betting on Barcelona 100/3.0 and 100/1.6 for Real Madrid.
€100 / 3 + €100 / 1.60 = €33.33 + €62.50 = €95.83
In the example above there are only two outcomes of the sporting event. But arbitration can be made with any type of bet and the sport as long as you can cover all possible profit.
Another very popular type of arbitrage is related to "purchase" of a profitable factor for a bookmaker and "selling" an arbitrary betting exchange.
There are different variation of arbitrage: Betting arbitrage, miraclebets, surebets, sports arbitraging and etc.