+150: Bet $100 to win $150 profit. The "+" means underdog.
−200: Bet $200 to win $100 profit. The "−" means favorite.
The bigger the number, the bigger the gap. −500 is a heavy favorite. +500 is a big underdog.
Multiply your stake by the decimal. $100 × 2.50 = $250 total return ($150 profit). Decimal odds are used in Europe, Australia, and Canada. They're the easiest to understand: higher number = bigger payout = less likely to happen.
3/1 (said "three to one") means you win $3 for every $1 bet. 1/4 means you win $1 for every $4 bet (heavy favorite). Used mainly in the UK and Ireland. Horse racing still uses fractional odds exclusively.
Odds represent probability. −200 = 66.7% implied probability. +200 = 33.3%. The bookmaker's margin (vig/juice) means the probabilities always add up to more than 100%. That's how they make money — the house always wins long-term.
Moneyline: Who wins the game. Simple.
Spread: Team A −3.5 means they must win by 4+. Used in football to even out mismatches.
Over/Under: Total goals scored. Over 2.5 means 3+ goals to win.
Parlay: Multiple bets combined. Higher payout, much lower probability. Bookmakers love parlays because they almost always win.