They are a very common type of betting. Consist predict whether the total score a game (football, basketball, handball.) Exceed a certain amount, which is usually decimal (completed in '5) to avoid the void. (Void means that just the number of points corresponding to the line of over / under, then the bet is taken for canceled and returned the amount risked).
In football the most common is the over / under 2.5 goals, ie if BETWEEN THE TWO TEAMS mark more (over) or less (under) of 2.5 goals. It is important to say that is not paid by the parties equally over and under, contrary to what we said with the handicap, who stood to equalize both quotas, and therefore possibilities.
The bookie assesses goals scored and received by each team and on that basis the quotations marks. There are leagues in which they score more goals per game (Germany, Netherlands.) And other leagues you are marked minus (Italy, France.). In some houses also offer other lines that are not 2.5, but this is the most common.
In basketball and handball usual there is no line, it depends on the match markers fluctuate much more than they usually do in football. However, in the over / under, is very difficult to find value bets, as there is less subjective errors circumstances causing the odds makers. (The people in charge of putting quotas).
There strangest bets also work with this type of forecasts, such as the number of cars of Formula 1 Grand Prix finish, usually placed in 16.5 line and circuit function quotes, weather. They can also refer to individual actions, such as annotation of a basketball player, there is plenty of bets of this kind. After winning bets, may be the most popular as they can be guided to statistics quite well, unlike the case with other forecasts. Therefore, there are many systems that give us picks based on a program designed by a gambler according to statistical criteria.