Put simply, it's a bet on which team or player will win a game or contest, regardless of the final margin. Unlike other types of bets, such as a point spread bet, a parlay or a teaser, moneyline bets keep it simple: if you bet on the winning side, you'll cash your ticket.
The method for finding arbitrage opportunities entails looking for significantly differing odds on the same sporting event. If the odds differ greatly enough, there is a reasonable chance for arbitrage. A betting calculator will tell you how much opportunity is available. It helps to look at some real-world examples.