An exacta is a type of bet where you pick the horses who will finish first and second, in that order ("exact").

The cast of La Medida Exacta - 2011 includes: Constantino Caso as Contador

exacta

Exacta

Simply put, an exacta box is playing your selected horses to come in 1st and second. This is the same as a quinella wager except that to play the exacta box you would need to play two separate bets. Example: If you likes #1 and #4 to be first and second you could either play a quinella which would pay you as long as they were indeed 1st and 2nd no matter which finished 1st. In an exacta you are playing them to finish ONE WAY ONLY (#1 then #4 OR #4 then #1). If you box your exacta you are then playing both of those combinations together and thus pay for two different exacta plays (1-4 and 4-1). If you feel very strongly one horse is better than the other you might play a straight exacta meaning they must finish in the order you picked. If they were to finish the opposite way, your straight exacta would lose but your exacta box would have won as would an quinnella play involving the two.