Betting on sports involves predicting the outcome of (normally) a sporting event. This could either be the final result or any event that occurs within the game. The odds (or price) that you take are set by the company and represents their idea of an event occurring.
To win you simply have to be right, although there is no such thing as a guaranteed winner. Even when backing a big favorite, winning is never guaranteed as sports ultimately are just too unpredictable.
Unlikely outcomes are not just possible, they actually happen quite regularly.
So how do 'odds' work?
The odds are a representation of how likely the bookmaker thinks that something will happen. The shorter the price, the more likely it is to occur and therefore the lower the winnings if you are successful.
Odds can be displayed as either American, decimals or fractions, they all equate to the same chance of winning they are just displayed differently.
For example +200, 2/1 and 3.00 are the same. If you placed $1 on this price and you were lucky enough to win you would get a total of $3 back, your original $1 risk and your winnings of $2.
Some more examples:
$1 @ +150 (6/4, 2.5) would return $2.50 in total if you won. That is your $1 risk and your winnings of $1.50.
$1 @ -150 (4/6, 1.66) would return $1.66 in total if you won. That is your $1 risk and your winnings of $0.66.
$1 @ +1000 (10/1, 11.00) would return $11 in total if you won. That is your $1 risk and your winnings of $10.
But remember chance and unpredictability play a part so the outcome that people are expecting doesn't always happen, the favorite might get beat, the outsider that no one thought could win might just have its day in the sun. So never bet more than you can afford to lose, gamble responsibly and within your means.