Decimal odds show the total return per unit staked, stake included: at 2.50, a £10 bet returns £25 (£15 profit plus the £10 stake). They are the default across Europe and the easiest format for calculation, which is why this site uses them everywhere.
Fractional odds (6/4) show profit relative to stake — win £6 for every £4 staked — and remain traditional in the UK. American odds use a plus or minus sign: +150 means a $100 stake wins $150; −200 means you must stake $200 to win $100. Converting: decimal = fractional + 1; positive American = (odds/100) + 1; negative American = (100/|odds|) + 1.
Worked example
Odds of 2.50, 6/4 and +150 are identical prices with an implied probability of 1 / 2.50 = 40%.