Betting on sports doesn't have to be all about picking a winner. In fact, a lot of bettors do very well while only rarely betting on one team or another. Instead they bet on totals, doing something that is also called over / under betting. Basically, over / under betting is betting on the number of points, runs or goals the two teams in a game are going to combine to score.
How it works is pretty simple. The oddsmakers will set a total for each game. The job of the bettor is to decide if the two teams involved in the game are going to score more points than the total (or 'go over'), or score fewer than the total (or 'go under'). Typically, the price for either the under or the over is -110, which means that a bettor would have to bet $110 to make a profit of $100 on a winning bet. The bookmakers can adjust that price if bettors are betting too much on one side and not enough on the other.
Over / under betting is usually available for every game in the NBA, MLB, NFL, and pretty much every other North American sport. They all work the same basic way, though the totals are obviously much higher for the NBA than they are for football, and higher for the NFL than baseball. Each sport also requires a slightly different approach to be successful.
We'll start with the NFL because it's the most popular sport to bet on in North America. The totals in football can vary wildly over the course of a season - you can see totals in the low 30s or the high 50s depending upon the teams involved. Successful over / under betting in the NFL requires the consideration of many factors that can affect the score of a game including the quality of the offenses and defenses of both games, how the offenses match up with the opposing defenses, the health of the key starters for both teams, the form that the two teams have, whether the game is played indoors or out, the weather and wind conditions of the game, what has historically happened when the two teams play, and more. Some teams have a tendency to go under more often than most, and they will consequently face generally low totals. Others, like the Colts in their prime, score a ton of points and don't play a lot of defense, so they face much larger totals and tend to go over more often.
Over / under betting in the NBA is harder because it deals with much bigger numbers, but it has the benefit of having fewer factors to worry about. Totals can, at times, be well over 200 for a game if both teams are offensive minded, though much lower totals are also possible in defensive battles. There are fewer player matchups to worry about in basketball than football, and you don't have to worry about things like weather, wind or field conditions. You also have the advantage of seeing teams play many more games than in football, so more significant trends can emerge. That doesn't mean that over / under betting in the NBA is easy, but for some people it works well.
Betting the over / under in baseball takes a very different mindset than the previous two sports. The totals are usually so low - typically between five and 12 - that one run can make a big difference. Many of the same things are applicable to baseball as to football, but by far the single biggest factor in betting baseball totals are the starting pitchers. A team can very likely allow just a run or two if they have a top ace on the mound, while the same team could be likely to allow several runs if they are forced to start a fifth starter or a pitcher from the bullpen or the minors if they have injury problems.
Over / under betting is also almost always available on college football and basketball. The same basic factors are involved in college as the pros, though the totals are often higher in college football than the NFL, and always lower in college basketball than the NBA because they play shorter games. In college basketball the style of play is also more of a factor because the same team is sometimes capable of scoring 50 points or 90 in a game depending upon their style and the style of their opponents. NHL bettors can also bet totals, and they have to focus on the quality of the goaltenders as they play such a huge role in hockey.