For a successful Exacta or Perfecta bet you have to pick the winner and the second place correctly. This means that you choose two horses and nominate one as the winner and the other as runner-up. Although the bet includes two horses it is placed as a single bet with a single wager, not with an individual wager for each horse.
You will only win this bet if your horses cross the line in the right order. For example, you have placed an Exacta/Perfecta bet on MacFlurry and LookOverThere, nominating MacFlurry as the winner and LookOverThere as second place. If your horses arrive in this order, MacFlurry first and LookOverThere second, you have won your bet. However, if your horses arrive first and second in the wrong order, i.e. LookOverThere first and MacFlurry second, you lose even though you have picked the first two horses correctly.
In an Exacta/Perfecta bet, the order in which you nominate your two horses is crucial. Also, both of your horses have to occupy the positions you nominated. I.e. if MacFlurry comes first but LookOverThere is in third place, you have won nothing even though you have picked the winner of the race. The Exacta/Perfecta bet pays off only in the exact constellation you nominate on your betting slip.
The odds for an Exacta/Perfecta bet depend on the number of horses starting in a race. In a race of five horses it is, of course, more likely to pick a correct Exacta/Perfecta than in a race of twenty. The more horses start in a race, the higher the pay-off for a Exacta/Perfecta bet.
This makes perfect sense considering that the less likely you are of winning, the more incentive has to be offered to make risky betting attractive. The easier options - like the Place bet and Show bet, or an Exacta/Perfecta bet in a four horse race – have a higher chance of paying off, which is why they will always offer a lower pay-off than high risk bets.