American odds, also called moneyline odds, use a number that is positive or negative. A negative number such as -200 is the stake needed to win 100 in profit, and it marks the favorite. A positive number such as +150 is the profit from a 100 stake, and it marks the underdog. The same price can be shown in decimal or fractional form, and the implied probability is the same in every format.
Example
+150 equals decimal odds of 2.50 and an implied probability of 40%. -200 equals decimal odds of 1.50 and an implied probability of about 66.7%.