When the referee reaches into his pocket after a foul, players wait, fearing that they may see the dreaded red card. The red card signals that a player has committed one of the most serious offenses in the game or that he has received two cautionary yellow cards during the game already. If you receive a red card there will be serious consequences for you and your team.
You're Outta There
A red card is used to signal what is known as a "sending off offense." This means that if you receive a red card while you are a player on the field, you will be sent off and cannot return to the game, nor can you stay on the sidelines. According to the rules you must leave the vicinity of the field and the technical area -- the area surrounding the team's bench.
If you are sent off as a player on the field, you cannot be replaced by a substitute. This means that your team must play shorthanded -- assuming that your opponent fields a full squad. This can be problematic if your team is already playing shorthanded. A team must have at least seven player to play. If you are sent off from the field when your team only has seven players, then the game is abandoned.
If your red card offense stops play then your team will be penalized at the restart of play. The opposing team will receive a direct free kick. On a direct free kick the kicker can score directly without any other player needing to touch it -- as opposed to an indirect free kick which must touch a second player before a goal can be scored. If you commit a red card offense in the penalty area in front of your own net the opposing team will be granted a penalty kick.
Off the Field
Substitutes can also be shown a red card and sent off if they commit a serious offense. Players can also be carded before the opening kickoff, during the halftime break and even after the game has ended until the referee leaves the field of play. If a player is sent off when he's already off the field he must leave the field area and cannot return. Because the player is already off the field it will not affect the number of players on the field.