Soccer Law 3: The Number Of Players

Players, equipment and officials

Each team consists of a maximum of eleven players (excluding substitutes), one of whom must be the goalkeeper. Competition rules may state a minimum number of players required to constitute a team; this is usually seven. Goalkeepers are the only players allowed to play the ball with their hands or arms, but they are only allowed to do so within the penalty area in front of their own goal. Though there are a variety of positions in which the outfield (non-goalkeeper) players are strategically placed by a coach, these positions are not defined or required by the Laws.

The basic equipment players are required to wear includes a shirt, shorts, socks, footwear and adequate shin guards. Players are forbidden to wear or use anything that is dangerous to themselves or another player (including jewelry or watches). The goalkeeper must wear clothing that is easily distinguishable from that worn by the other players and the match officials.

A number of players may be replaced by substitutes during the course of the game. The maximum number of substitutions permitted in most competitive international and domestic league games is three, though the number permitted may be varied in other leagues or in friendly matches. Common reasons for a substitution include injury, tiredness, ineffectiveness, a tactical switch, or as a defensive ploy to use up a little time at the end of a finely poised game. In standard adult matches, a player who has been substituted may not take further part in the match.

A game is officiated by a referee, who has "full authority to enforce the Laws of the Game in connection with the match to which he has been appointed" (Law 5), and whose decisions are final. The referee is assisted by two assistant referees. In many high-level games there is also a fourth official (and in the world cup a fifth official), who assist(s) the referee and may replace another official should the need arise.

Source: Wikipedia