There are three major formats of sports betting odds. Below is a short description of the most popular odds systems.

British - Fractional Odds.
Fractional odds is the default odds format offered by bookmakers in the United Kingdom and Ireland. These odds are indicated as a fraction, and represent the net payout earned relative to the stake. If a bet wins the initial stake is returned with the payout.
For example odds of 4/1 imply that the bettor stands to make £100 on a bet of £25. If the odds are 1/4 the better will make £5 from a £20 stake. The bettor will also always receive their stake back, so a successful bet of £25 at 4/1 will actually pay out £125 - which is £100 plus the return of the original £25 bet.

European - Decimal Odds.
Decimal odds are most popular in Europe, Australia and Canada. Most online sportsbooks will offer the option of displaying their odds in decimal manner.
It indicates the number of dollars that would be returned if the bet wins including the initial stake.
Hence, decimal odds of 2.4000 imply that if a €1 bet were to win, €2.40 would be returned to the bettor (the €1 initial stake plus €0.50 in winnings), while decimal odds of 2.0000 imply that if a €1 bet were to win €2 would be returned to the bettor (the €1 initial stake plus €2 in winnings).

American Odds.
American odds work differently for favorites and underdogs, and are sometimes known as Moneyline Odds. These odds reflect the payout on a successful $100 bet, with the return of the stake not included in this figure.
US-style odds are quoted as either a positive or a negative number. A negative number represents the dollar amount you have to stake in order to win $100. If the odds offered on a particular event result are -210, it means that in order to win $100 you would need to bet $210. A positive number indicates the dollar amount that would be won from a $100 wager. If the odds offered on a particular event result are +150, it means that in order to win $150 you would need to bet $100.

American, Fractional and Decimal Odds Comparison