The group stage of a tournament is often used at the beginning, as a first round. Teams are put into a group and play the other teams in that group, in a league format. The number of groups and number of teams per group is based upon the number of teams in a tournament.
Teams generally play ech other twice: once home and once away; however, in some tournaments, there is only one match, played at a neutral ground (for example, the World Cup. The winner of a group is determined by the number of points earned by each team. In most cases, 3 points are given for a win, 1 for a draw and 0 for a loss. However, there are some tournaments where points are given for scoring goals of away win (for example, the Australian Pre-Season Cup). Usually, the winners (and is a lot of cases, 2nd place) will qualify for the next round of the tournament. Sometimes, teams one position below the qualification place qualify for a play-off. Qualification rules often varies widely, depending on the tournament.