When betting online there is normally just two types of betting odds to choose from, either fractional odds or decimal odds, although bear in mind other odd types do still exist.
For those of you that have bet before, you will most likely be familiar with fractional odds. Favoured by UK and Irish bookies alike, and with strong ties to horse racing, fractional odds show the net return that the bettor could potentially win, relative to their given stake. For instance fractional odds of 3/1 (three-to-one) indicate that the bettor could make a £3 profit for every £1 staked, i.e £3 for every £1. Note when betting with fractional betting odds, your stake is always returned along with your winnings, so £1 bet at 3/1 would return £4 in total if your bet was to win.
All prices quoted on a betting exchange are in 'Decimal' Odds. Decimal Odds differ from the Odds traditionally quoted by UK bookmakers in that they include your stake as part of your total return. If you place a bet of £10 at Decimal Odds of 4.0 and win, then your total return (including stake) is £40. In a traditional bookmaker this would be quoted as 3/1, returning to you winnings of £30 plus your original stake of £10.