Very simply, the Over and the Under are wagers that you make on the Total. The Total is the number of points scored in a contest by both teams or players combined. To give an example, here is what the typical Over and Under betting lines might be for a major league baseball game;
New York Yankees vs. Detroit Tigers
9½ (-125)o (+105)u
In the above example, the Total is 9½, the Over is (-125)o, and the Under is (+105)u. The sportsbook is guessing that the number of runs scored by the Yankees plus the number of runs scored by the Tigers will be about 9 or 10. They list the Total as 9½ however for reasons also discussed in the Point Spread article. The setting of the Total as a fraction means that anyone betting on the Total will either win or lose. That’s because you cannot score ½ point in baseball.
If you believe that the total number of runs scored by the Yankees and the Tigers will be 10 or more then you would bet the Over of (-125). You would risk $125 for the chance to win $100 in this case. On the other hand, if you think that the total number of runs will be 9 or less then you would bet the Under of (+105). If you did that you would be risking $100 for the chance to win $105.
Assume that the final score of the baseball game is New York 6, Detroit 4. The total number of runs scored in the game is thus 10. If you had bet $125 on the Over you would have won $100 from your bet because the actual total of 10 is “over” the Total of 9½ runs listed in the betting lines. But if you had bet the Under, you would have lost. Of course, if the score was New York 5, Detroit 4, then any player who bet the Over would have lost. But betting $100 on the Under would have won $105 since the actual total of 9 would be “under” the betting lines Total of 9½ runs.
Over and Under bets can be made on just about any sport. Boxing matches often have Over/Under bets on the number of rounds that the boxing match will last. And most other sports also allow you to bet the Over or the Under based on the score halfway through the game. You can even bet the Over and the Under on the total points scored by just one team, although these would usually appear under “Team Props” rather than under the betting lines for the game.