The three main types of betting odds are fractional (British) odds, decimal (European) odds, and American (moneyline) odds. These are simply different ways of presenting the same thing and hold no difference in terms of payouts. They represent the probability of an event to happen and therefore enable you to work out how much money you will win if your bet wins.
Fractional Odds are mostly used by British and Irish bookies. A fractional listing of X/Y (X-to-Y) odds would mean that you win X against every Y you wager (in addition to receiving the amount you wagered back) in addition to the profit (X). Total Return = [Stake x (Numerator/Denominator)] + Stake where numerator/denominator is the fractional odd.
Decimal Odds, are used in continental Europe, Australia, New Zealand and Canada. The decimal odds number represents the amount one wins for every $1 wagered. Total Return = Stake x Decimal Odd Number
American (Moneyline) Odds are used in the United States. The odds for favorites are accompanied by a minus (-) sign, indicating the amount you need to stake to win $100. Meanwhile, the odds for underdogs are accompanied by a positive (+) sign, indicating the amount won for every $100 staked.
To sum it up: Fractional odds are the ratio of the amount (profit) won to the stake; Decimal odds represent the amount one wins for every $1 wagered; and American odds, depending on the negative or positive sign, either indicate the amount one needs to wager to win $100 or the amount one would win for every $100 staked.
Now, there are also different types of bets:
Moneyline is the easiest type of bet. In a Moneyline bet, you bet simply on who will win the game. Simple as that. This form of betting odds focuses just on who will be the outright winner. Here is an example. Evil Genius odds are 1.66, while PSG.LGD odds are 2.4. The first thing you need to pay attention to, is if any odds is above 2. The team with 2 or above odds, is the underdog and considered to be the loser. In this case, Team B. This means they can potentially earn you a higher payout if they manage win. Depending on whether it's below or above 2, it's got different meanings. With odds set at 2.4, Team B offers a $140 payout for every $100 bet on it. An odds below 2 means the team is favored to win. The number indicates how much you would need to wager in order to win $100.
Handicap betting in Dota 2 works the same as handicap betting in traditional sports. When it comes to Dota 2, rather than score as it would be when betting on football matches, the handicap represents the game score in a best of series. So take a best-of-three match in Dota 2, you bet on Evil Genius with +1.5 handicap bet. PSG.LGD beats Evil Genius 2-1 and since you bet +1.5 on Evil Genius, 1.5+1=2.5 which is higher than the 2 of PSG.LGD so you win your bet. However, if Evil Genius failed to win a single game and lost 2-0 then you would lose your bet because 0+1.5=1.5 and that is less than 2.
This works the same for negative handicap betting, if you bet on Evil Genius with a -1.5 handicap bet and they beat Team B 2-1 then you would lose the bet as 2-1.5=0.5 which is lower than 1. But if they managed to win 2-0 then you would win the bet as 2-1.5=0.5 which is higher than 0. Handicap betting can be used for best-of-two matches with the +/- decimal 0.5 and best-of-five matches with a +/- 2.5 decimal.
Total Maps Played is a bet on whether the total number of maps played in the series is over or under the specified strike. In a best of 3, for example, this is a bet on whether or not Map 3 gets played - the Under wins when either team wins 2-0, and the Over wins when the third map gets played.
A Map Winner is a bet on who will win a specific map in the series.
The Kills spread is just another form of betting odds, quite similar to a Moneyline bet. The difference is that in addition to predicting who will win the game, the point spread includes a margin of victory. In certain cases such as in Dota 2 and Counter-Strike GO, there is a huge difference skillwise between the two teams playing. In those cases, betting the moneyline is quite obvious, because it is clear that one team should beat the other. That’s when the point spread comes into play, as the oddsmakers will set a line where there is a margin of victory. For example, imagine that two teams, Evil Genius and PSG.LGD, are facing each other. Evil Genius has odds of +10.5 while PSG.LGD odds are -10.5. In this case, PSG.LGD is favored to win. Just like in the Moneyline bet, the negative sign before the number indicates who is expected to win. The number represents the expected margin of victory. If you bet on PSG.LGD, you need them to win by more than 10.5 points for you to cover your bet. That means they need to win the game by 11 points or more. If you bet on Evil Genius, that means they can lose by 10 points or less, or win the game, and you still win your bet. It’s a way of evening the playing field, even if one team is highly favored to win over the other one. If, during the actual game, the margin of victory lands exactly on the spread, it is called a push or a tie, and no one wins the bet. In this event, you would simply get your money back. Point spreads will also have moneyline odds attached. Just like in a Moneyline bet, this number indicates what the payout will be in the case of a win. For example, the spread and odds for PSG.LGD might be (-10.5, 1.66). This means that PSG.LGD is favored to win by more than 10.5 points. Simple, right?
The First Blood bet is a bet on which team will score the first kill on a map. This will be graded using the in-game scoreboard - so any hero denies will not count.
We have Race to 5, 10, 15, and 20 Kills bets that will be available. Whichever team reaches certain kill score on the scoreboard first, wins the bet. Only kills that show up on the scoreboard count (i.e. hero denies do not count).
The Team to Slay the First Roshan is a bet on which team can first kill the neutral creep Roshan. Killing Roshan is an important objective in Dota 2 as he drops valuable rewards that can help the team to win the map.
Prop bets, or “proposition bets”, can be on literally just about anything. From an individual player’s performance in a particular game to who will win an event. Prop bets generally refer to anything that is not directly tied to the outcome of the game. They can be over/under bets on how many times a player does something, or they can be bets on which team will do better in general on a certain topic. For example, in a Dota 2, the oddsmakers may set the odds for which team will get more kills. In the end, a prop bet can be on just about anything. Whatever an oddsmaker can imagine, can become an interesting prop bet.