What is Arbitrage Betting?

Legal Definition
Betting arbitrage, miraclebets, surebets, sports arbitraging is a particular case of arbitrage arising on betting markets due to either bookmakers' different opinions on event outcomes or plain errors. When conditions allow, by placing one bet per each outcome with different betting companies, the bettor can make a profit regardless of the outcome. Mathematically arbitrage occurs when there are a set of odds, which represent all mutually exclusive outcomes that cover all state space possibilities (i.e. all outcomes) of an event, whose implied probabilities add up to less than 1. In the bettors' slang an arbitrage is often referred to as an arb; people who use arbitrage are called arbers.

Arbitrage betting involves relatively large sums of money, given that 98% of arbitrage opportunities return less than 1.2%. The practice is usually detected quickly by bookmakers, who typically hold an unfavorable view of it, and this can result in half of an arbitrage bet being canceled. Arbitrage betting is almost always insufficiently profitable due to detection, unreliable betting websites, limiting of stakes, hackers, and scammers that use high percentage arbitrages to trick bettors into providing security credentials.

Bookmakers generally disapprove of betting arbitrage, and restrict or close the accounts of those who they suspect of engaging in arbitrage betting. Although arbitrage betting has existed since the beginnings of bookmaking, the rise of the Internet, odds-comparison websites and betting exchanges have made the practice easier to perform. On the other hand, these changes also made it easier for bookmakers to keep their odds in line with the market, because arbitrage bettors are basically acting as market makers.

In Britain, a practice has developed in which highly experienced "key men" employ others to place bets on their behalf, so as to avoid detection and increase accessibility to retail bookmakers and allow the financiers or key arbitragers to stay at a computer to keep track of market movement.

Arbitrage is an extremely fast-paced process and its successful performance requires lots of time, experience, dedication and discipline, and especially liquidity.
-- Wikipedia