To win a quinella, you must select the horses that finish 1st and 2nd. You don't have to pick which horse will win and which one will come 2nd, just as long as you have the first and 2nd horses in your bet. Let's say you box three horses in a quinella, numbers 1, 4 & 6. If the result of the race ends up being say 6, 1 & 10 you would win, but if it was 6, 10 & 1 you would lose.

The difference between a quinella and an exacta, is that with an exacta you must choose which horse you think will win, and which horse will come second. It's harder to win than a quinella but generally pays more as a result.

Does that help?