What are fouls?
Occasionally play is stopped by an official when a player commits a foul. A foul is an act of illegal conduct by a player towards another player. The rules leave this open-ended by defining a foul to be an act of careless, reckless, or excessive force as described by FIFA. Some common fouls you'll see during a game are the result of a poorly executed tackle or slide tackle. Fouls are an infraction in soccer resulting from a player breaking the rules defined in the laws of the game. A referee will call one on a player who violates a rule and is careless, reckless, or uses excessive force. Other infractions may result from unsportsmanlike conduct.
Who can call a foul?
In soccer, referees call fouls on player either with a whistle by the main official or the waving of a flag from the sideline official. Next, the main official will stop play and assess the result of the foul with a penalty card or free kick, or continue play with an advantage.
If a foul is committed during the course of play, the referee will either stop play and award a free kick, or allow play to continue with the advantage rule. Depending on the severity of the foul, a penalty card may also be given to the offender.