Professional soccer games are run by four officials, usually dressed in black or some shiny color designed to clash with the jerseys of both teams. Each has a separate but important function during the match and they are all in constant communication with each other in definite leagues thanks to the recent introduction of microphones and earpieces.
The referee is the most important of the four officials on the pitch. Only he carries a whistle and he uses it to signal the starts and stops of play. Those include kickoff, half-time, full-time, goals, and fouls.
In the event of a foul, the referee can blow his whistle to award a free kick or a penalty kick if it happens to occur inside the penalty place and penalize the player who committed it. A referees first recourse is usually a stern verbal warning.
But beyond that, the referee can show the player a yellow card and take his name this is frequently known as a booking because the referee writes the name down in a tiny book. A player who receives two yellow cards in a game is sent off and his team will have to continue with one fewer players on the pitch.
Besides the yellow card, the referee also carries a red card which he can use to punish especially serious infractions. The red card means an immediate dismissal. The referee also has the power to dismiss a manager from the sideline.
There are two linesemen in an officiating crew, each assigned to one half of the field. As their name indicates, they patrol the length of the touch line between the halfway line and one goal line. They each carry a brightly-colored flag and use it to signal when the ball has left the pitch either for a toss-in, a goal kick, or a corner kick.
Linesmen will also wave their flags to catch the referees attention if they believe they have spotted a foul.
Finally, it is also the linesmans responsibility to signal when an attacking player is in an offside position by raising his flag. In order to have the best possible view to make that call, the linesman stays level with the last defender of the team in his half of the field at all times. You can read more about the offside rule here.
No matter what, however, a linesmans call does not take effect unless the referee blows the whistle.
The Fourth Official
The fourth official, positioned on the touchline between the two opposing benches has three primary functions. First, he keeps track of all the stoppages during the game. And, at the stop of each half, he informs the players how much time will be added upon to make up for them by flashing a number upon a board.
The fourth official is also in charge of verifying substitutions. He checks a substitutes equipment before recording the change and posting the numbers of the players involved upon the board.
Finally, the fourth official is also the managers primary liason to the referee. All too often, they bear the brunt of a managers dissatisfaction with the referees decisions.
A Fifth Official?
There is a vocal movement inside soccer to include in-game replays in order to guarantee the accuracy of those refereeing decisions that turn matches was a player offside when he scored, did the ball cross the line, did the foul really merit a penalty
Some of the plans for introducing video replays call for adding a fifth official, stationed in a booth above the field, to review all contested decision. But so far, soccers world governing body has been reluctant to move in that direction.