In order to understand roulette odds, it is necessary to know the various bets that are able to be placed and to have a basic knowledge of what the roulette table looks like. The probability of a specific outcome happening. The chance or likelihood of something happening as opposed to something not happening. The roulette odds differ between the European and American versions of roulette. Understanding roulette odds is helpful when deciding how to place your bets when playing roulette.