During a regular soccer game, each team should have a maximum of eleven players (excluding substitutes) on the field during play. The goalkeeper is one of the eleven players on the field.
While eleven on the field is the most desirable number of players, competition rules allow a team to play with as few as seven players. Any number fewer then seven does not constitute a team. The Goalkeepers are the only players allowed to play the ball with their hands or arms, as long as they are within the penalty area in front of their own goal. Most coaches will strategize the positions of the players on the field, however the line ups are not specifically defined within the rules of the game.
In soccer (or football as it is called by the majority of the world), the uniform is referred to as a kit. The kit consists of a shirt, shorts, socks, footwear and adequate shin guards. Headgear is neither required or restricted and many players wear protection to prevent head injuries. Watches and jewelery are not allowed to be worn by players in any capacity. (Any thing that can cause damage to oneself or another player is also forbidden on the field.)
The goalkeeper is required to wear something different from the rest of the team so that it is easy for both the game officials and players on the field to identify him/her easily. Substitutions are allowed in all games, the number of substitutions many vary depending on the league and level of play. For example, the maximum number of substitutions permitted in most competitive international and domestic league games is three, however this requirement does not apply in friendly competition or for youth players in most cases. Substitutions can be made for any reason at the discretion of the coach, however the most common reasons for a substitution include injury, tiredness, ineffectiveness, and for tactical purposes. In most competitive international and domestic league games, a player who has been substituted may not return to active play during the game, however in most youth soccer leagues, this does not apply.
If any event results in fewer than seven field players on the field (illness/injury) the game should be ended at that time. The rules relating to this situation are applied on a case by case basis depending on the rules of the league or club. Games are officiated by a referee, who has final say on any decision regarding the match. At more advanced levels, the referee is assisted by two assistant referees or linesmen and it the highest levels, there is also a fourth official who assists the referee and may replace another official should the need arise.