Rules of the game

The court
The game is played on a volleyball court 18 meters long and 9 meters wide, divided into two 9 x 9 meter halves by a one-meter wide net placed so that the top of the net is 2.43 meters above the center of the court for men's competition, and 2.24 meters for women's competition (these heights are varied for veterans and junior competitions).
There is a line 3 meters from and parallel to the net in each team court which is considered the "attack line". This "3 meter" (or 10 foot) line divides the court into "back row" and "front row" areas (also back court and front court). These are in turn divided into 3 areas each: these are numbered as follows, starting from area "1", which is the position of the serving player:

After a team gains the serve (also known as siding out), its members must rotate in a clockwise direction, with the player previously in area "2" moving to area "1" and so on, with the player from area "1" moving to area "6" (see also the Errors and faults section).
The team courts are surrounded by an area called the free zone which is a minimum of 3 meters wide and which the players may enter and play within after the service of the ball. All lines denoting the boundaries of the team court and the attack zone are drawn or painted within the dimensions of the area and are therefore a part of the court or zone. If a ball comes in contact with the line, the ball is considered to be "in". An antenna is placed on each side of the net perpendicular to the sideline and is a vertical extension of the side boundary of the court. A ball passing over the net must pass completely between the antennae (or their theoretical extensions to the ceiling) without contacting them.

The ball

The volleyball is made of leather or synthetic leather and inflated with compressed air. According to FIVB regulations:
Its circumference is 65-67 cm and its weight is 260-280 g.
Its inside pressure shall be 0.30 to 0.325 kg/cm2 (4.26 to 4.61 psi) (294.3 to 318.82 mbar or hPa).

Game play
Each team consists of six players. To get play started, a team is chosen to serve by coin toss. A player from the serving team (the server) throws the ball into the air and attempts to hit the ball so it passes over the net on a course such that it will land in the opposing team's court (the serve). The opposing team must use a combination of no more than three contacts with the volleyball to return the ball to the opponent's side of the net. These contacts usually consist first of the bump or pass so that the ball's trajectory is aimed towards the player designated as the setter; second of the set (usually an over-hand pass using wrists to push finger-tips at the ball) by the setter so that the ball's trajectory is aimed towards a spot where one of the players designated as an attacker can hit it, and third by the attacker who spikes (jumping, raising one arm above the head and hitting the ball so it will move quickly down to the ground on the opponent's court) to return the ball over the net. The team with possession of the ball that is trying to attack the ball as described is said to be on offense.
The team on defense attempts to prevent the attacker from directing the ball into their court: players at the net jump and reach above the top (and if possible, across the plane) of the net in order to block the attacked ball. If the ball is hit around, above, or through the block, the defensive players arranged in the rest of the court attempt to control the ball with a dig (usually a fore-arm pass of a hard-driven ball). After a successful dig, the team transitions to offense.
The game continues in this manner, rallying back and forth, until the ball touches the court within the boundaries or until an error is made.

Errors and faults
The ball lands out of the court, in the same court as the team that touched it last, under the net to the opposing team's court, or the ball touches the net "antennas." The ball also may not pass over or outside the antennas even if it lands in the opponents' court.1
The ball is touched more than three times before being returned to the other team's court.2
The same player touches the ball twice in succession.3
A player "lifts" or "carries" the ball (the ball remains in contact with the player's body for too long).
A player touches the net with any part of his or her body or clothing while making a play on the ball (with the exception of the hair).
The players of one team do not manage to touch the ball before the ball lands in their half of the court.
A back-row player spikes the ball while it is completely above the top of the net, unless he or she jumped from behind the attack line (the player is, however, allowed to land in front of the attack line).
A back-row player participates in a completed block of the opposing team's attack (completed means at least one blocker touched the ball).
The libero, a defensive player who can only play in the back row, attempts a block or makes an "attacking hit", defined as any shot struck while the ball is entirely above the top of the net.
A player completes an attack hit from higher than the top of the net when the ball is coming from an overhand finger pass (set) by a libero in the front zone.
A player is not in the correct position at the moment of serve, or serves out of turn. This type of foul is related to the position currently occupied by the players (see the table in the Equipment section). When ball is served, players can place themselves freely on the field (e.g. a "back-row" player can be close to the net) so long as they obey the following rules: The area "1" player must be behind the area "2" player and to the right of the area "6" player. The area "6" player must be behind area "3" player, to the left of area "1" player and to the right of area "5". The area "5" player must be behind the area "4" player and to the left of the area "6" player. Symmetric rules must be respected by the front-row players (those in areas "2", "3" and "4").
When hitting, a player makes contact with the ball in the space above the opponent's court (in blocking an attack hit, this is allowed).
A player touches the opponent's court with any part of his or her body except the feet or hands.4
When serving, a player steps on the court or the end line before making contact with the ball.
A player takes more than 8 seconds to serve.
At the moment of serve, one or more players jump, raise their arms or stand together at the net in an attempt to block the sight of the ball from the opponent (screening).5
A player blocks the serve or attacks the serve when the ball is in the front zone and above the top of the net.
Notes:
1 If the ball passes outside the antennas on the first contact for the team, e.g. as the result of a bad pass or dig, a player is allowed to go after the ball as long as he or she does not touch the opponent's court and the ball travels back to his or her team's court also outside the antennas.
2 Except if a player blocks (touches a ball sent over the net by the opposing team, while reaching above the top of the net) a ball that stays in the blocker's side of the net. In such an instance the blocker may play the ball another time without violating the rule against playing the ball twice in succession. If the ball is touched during a block, that contact is not considered one of the team's three contacts.
3 At the first hit of the team, the ball may contact various parts of the body consecutively provided that the contacts occur during one action. Also, when a player touches the ball on a block, he or she may make another play on the ball.
4 Penetration under the net with hands or feet is allowed only if a portion of the penetrating hands or feet remains in contact with or directly above the player's court or center line.[13]
5 Screening is only a fault if the players stand directly next to each other in a way that clearly impedes vision, and the serve is a low line drive over their heads. (This is a judgment call by the referee. Teams are generally given a warning before being sanctioned for screening.)

Scoring
When the ball contacts the floor within the court boundaries or an error is made, the team that did not make the error is awarded a point, whether they served the ball or not. The team that won the point serves for the next point. If the team that won the point served in the previous point, the same player serves again. If the team that won the point did not serve the previous point, the players of the team rotate their position on the court in a clockwise manner. The game continues, with the first team to score 25 points (and be two points ahead) awarded the set. Matches are best-of-five sets and the fifth set (if necessary) is usually played to 15 points. (Scoring differs between leagues, tournaments, and levels; high schools sometimes play best-of-three to 30; in the NCAA games are played best-of-five to 30.)
Before 1999, points could be scored only when a team had the serve (side-out scoring) and all sets went up to only 15 points. The FIVB changed the rules in 1999 (with the changes being compulsory in 2000) to use the current scoring system (formerly known as rally point system), primarily to make the length of the match more predictable and to make the game more spectator- and television-friendly.

The Libero
In 1998 the libero player was introduced internationally, the term meaning free in Italian is pronounced LEE-bah-ro (rather than lih-BEAR-oh); the NCAA introduced the libero in 2002. The libero is a player specialized in defensive skills: the libero must wear a contrasting jersey color from his or her teammates and cannot block or attack the ball when it is entirely above net height. When the ball is not in play, the libero can replace any back-row player, without prior notice to the officials. This replacement does not count against the substitution limit each team is allowed per set, although the libero may be replaced only by the player whom they replaced. The libero may function as a setter only under certain restrictions. If she/he makes an overhand set, she/he must be standing behind (and not stepping on) the 3-meter line; otherwise, the ball cannot be attacked above the net in front of the 3-meter line. An underhand pass is allowed from any part of the court.
The libero is, generally, the most skilled defensive player on the team. There is also a libero tracking sheet, where the referees or officiating team must keep track of who the libero subs in and out for. There may only be one libero per set (game), although there may be a different libero in the beginning of any new set (game).
Furthermore, a libero is not allowed to serve, according to international rules, with the exception of the NCAA women's volleyball games, where a 2004 rule change allows the libero to serve, but only in a specific rotation. That is, the libero can only serve for one person, not for all of the people for whom she goes in. That rule change was also applied to high school play soon after.

Recent rule changes
Other rule changes enacted in 2000 include allowing serves in which the ball touches the net, as long as it goes over the net into the opponents' court. Also, the service area was expanded to allow players to serve from anywhere behind the end line but still within the theoretical extension of the sidelines. Other changes were made to lighten up calls on faults for carries and double-touches, such as allowing multiple contacts by a single player ("double-hits") on a team's first contact provided that they are a part of a single play on the ball.