From DCFanguide

Everyone knows that the main rule is "You can't use your hands." In here, we'll try and explain some of the other key rules.

[edit] Length of Game

A game takes 90 minutes, separated into two 45 minute halves. There are no timeouts that can be called, although there are stoppages in play for injuries, substitutions, and to enforce fouls. The referee keeps track of these stoppages and allocates a small amount of additional time (usually between one and five minutes) at the end of each half to compensate for any stoppages in the previous half. The official time is kept by the referee on the field, any clocks in the stadium are unofficial and just there as a quick aide.

[edit] The Field

Unlike most sports, the size of the field is not fixed at one size. Instead the field has to be 110 to 120 yards in length and 64 to 75 yards wide. If you watch the game on TV or at another stadium, you'll notice that some fields are longer or narrower than the field at RFK.

[edit] Substitutions

Each team may make up to three substitutions. Once you leave the game, you can't come back. If you use all three substitutions, and someone gets hurt and has to leave the field, you play shorthanded.

[edit] Areas of the Field

1. Center Circle - Both halves start when one team kicks off the game from the center of this circle. After each goal, the team that was scored on will restart the game from here as well. Until the ball is kicked, players from the opposing team have to stay outside the circle.
2. Penalty Area - These large (18 yd x 44 yd) rectangles on either end of the field define the area in which a foul that normally is punished with a direct free kick becomes a punishable by a penalty kick. Also, the goalkeeper is allowed to handle the ball here. The little arc on the side of the penalty area near the center circle is not part of it.
3. Goal Area - These smaller (6 yd x 20 yd) rectangles on either end of the field define the area the defending team may use to restart play via a goal kick after the opposing team kicks the ball over the Goal Line but outside of the goal itself.
4. Touch line - Lines indicating the sides of the field. The entire ball must be over the entire line before it is "in touch." When this happens play is restarted by the team that didn't touch it last by means of a throw in.
5. Goal Line - Lines indicating the ends of the field. Balls going beyond the Goal Line but not into the goal are restarted by one of two means. If the defending team is the last to touch it the restart is a corner kick by the offensive team, taken within one yard of the corner flag. Defenders must stay ten yards away until the ball is kicked. If the offensive team is last to touch the ball then the restart is a goal kick taken from the goal area, usually by the goalkeeper.
6. Corner Flags - Small flags indicating where the Touch Lines and Goal Lines meet. Mainly useful in preventing the confusion over whether the ball rolled over the goal line or touch line.

[edit] Tackling and Fouls

1. Slide Tackles. In general, a "tackle" is when a defender takes the ball away from the other team. Since most professional soccer players can shield the ball from a defender with their body, a common defensive tactic is to slide in feet first to poke the ball away. This tactic, if done improperly, can easily result in a foul. Two common fouls are when the defender gets the player instead of the ball with his feet, or if the defender raises his feet up and clips the player even if he also pokes away the ball.
2. Shoulder Tackles. Another defensive tactic is to knock a player "off" the ball with your shoulder. The logic here is that the person closer to the ball generally is thought to be in possession of the ball and therefore the one getting fouled. Running shoulder-to-shoulder with another player puts both players approximately the same distance from the ball, and therefore they can collide with neither being called for a foul. If the defender is called for a foul when trying this, it's usually because they hit the other player from behind (as opposed to shoulder-to-shoulder), or pushed off with their arms when making contact.
3. Fouls. Fouls are generally divided into two sorts; fouls against the opposing team, such as pushing or grabbing (which result in a "direct kick") and infringements of the rules such as improperly throwing the ball in (which result in an "indirect kick"). The exception to the general guideline is touching the ball with your hand or arm; even though this is not against the opposing team, a direct kick is awarded if this happens. A direct kick can be kicked directly into the goal without another player touching it, giving the kicking team a great advantage when this type of foul occurs within 30 yards or so of the goal. An indirect kick requires (no surprise here) a second player (from either team) to touch the ball before it can be counted as a goal.
4. Penalties. If a foul occurs within the penalty area (see above) that would result in a direct kick, the fouled team is awarded a penalty kick. This is taken from 12 yards away from the goal, with only the opposing goalie allowed to be within the penalty area. At the professional level, this should almost always result in a goal, and hence there is always a lot of controversy when this type of penalty is called.
5. Advantage. Soccer is one of the few sports that allow play to go on after a foul has been committed if the team with the ball is still at an advantage. This serves to keep the game moving when there is nothing to be gained by stopping play and having a direct or indirect kick. The referee indicates that he has seen the foul but is allowing play to go on by holding both arms out straight in front of him (or her).
6. Yellow and Red Cards. A yellow card is an official warning to a player that they are doing something above the level of a normal foul. The referee will stop the game and hold the yellow card above his head in the direction of the offending player. This is also referred to as a "caution" or a "booking" (the latter refers to the fact that the referee writes the offending players number in his game "book"). There are seven official reasons for a yellow card, but it generally breaks down into overly rough play or just doing something really stupid (like harassing the goalkeeper as he is trying to release the ball). Usually in professional soccer yellow cards are given for overly aggressive play (officially called "unsporting behavior"). A red card means the offending player is ejected from the game, and his team is not allowed to substitute (so they will play down a man). There are also seven official reasons for this, but generally these are given for "serious foul play" (such as hitting an opponent or spitting on an opponent) or if one player gets a second yellow card in the same game. Not only does the ejected player miss the end of the game, but he cannot play in the team's next game.