skrilla is right.
say the O/U for a hockey game is 6.5. if you bet under and they score 6 goals, you win the bet. if they score 7, you lose, and so on.
by the way, if you end up winning on a ticket, you're good or real lucky. i've won twice in about 5 years in hockey - it's nearly impossible to predict.
ie. Atlanta winning in Dallas last night (Turco is out, but still...)