A sure bet, also known as an arb bet, or just an arb (short for arbitrage) is a form of bet where you cannot lose. Yes, you read that correctly. Sounds too good to be true? Well, in this case it really isn’t.
Bookmakers have different odds for many reasons and because there are now so many companies the differences can sometimes be quite pronounced. When they are sufficiently large that you can back all available outcomes and still make a profit, that is when you have a sure bet.