Over/under wagers task bettors with picking whether two sports teams will produce a final combined score will end up over or under a line set by the book. In a totals bet, it doesn't matter which team wins the game, or how many points each team scores.
A point spread bet tasks bettors with choosing which team will win the game with the point spread added to the final score. The total combined score doesn't matter in the case of a spread bet.