Betting odds come in three formats — decimal, American (moneyline) and fractional — that all express the same thing: the payout and the implied probability of a bet.
Decimal (2.50) is total return per unit staked, including your stake. American (+150 / −120) shows profit on $100 or the stake needed to win $100. Fractional (3/2) is profit-to-stake. They're interchangeable — just different notation.
+150 American = 2.50 decimal = 3/2 fractional = 40% implied. To convert American to decimal: positive → 1 + odds/100; negative → 1 + 100/|odds|. Decimal is the easiest to do math with, which is why sharp bettors prefer it.
Decimal is the clearest for calculating payouts and probabilities, which is why most tools and sharp bettors use it. American is standard at US books; fractional at UK books.
Use an odds converter, or convert everything to decimal first: it's total return per unit, so probability is just 1 ÷ decimal.