The following is a basic guide to the main rules, regulations and terms.

DURATION
A game consists of two halves of 40 minutes each with teams swapping ends at the break. The referee will add on any time lost due to stoppages.

PLAYERS
Only 15 players representing each squad are allowed onto the pitch at any time. A team may substitute two front-row (two props and the hooker) players and five from other positions in a match. A player who has been replaced must miss the remainder of that match.
There is a division between forwards and backs which nowadays pretty much only applies in line-outs and scrums when play is restarted after the ball has been buried on the field or gone out of touch.
The backs are the players not involved in line-outs and scrums. Should the forwards have possession of the ball, the backs will attack otherwise they defend and attempt to gain possession.

UMPIRES
There is one referee, who oversees the match. He stops and starts play with the use of a whistle. He also rules on scoring, infringements and players must have his permission to enter or leave the field. In contrast to other sports, rugby is characterised by an absolute respect to the referee.
The referee is assisted by the two touch judges, one on either side of the pitch. The touch judges stand behind the posts when there is a kick at goal. They indicate touch and decisions with the use of a flag.

SCORING
A player scores a try when he touches the ball down behind the opposition's try-line. Five points are awarded for a try, two points for a conversion, which is a kick between the goalposts and over the crossbar from a point in line with the place where the ball had been grounded behind the try-line.
Three points are given for a penalty, which is kicked from the point where the referee rules an infringement.
Players can also earn three points for a drop goal, which is kicked during play when a player drops the ball to the ground and kicks the rebound between the posts and over the crossbar.
A penalty try is awarded when the referee decides the attacking side would have scored had it not been for an infringement by the defenders. The conversion is taken from under the posts.

PENALTIES
The referee may caution and eject a player from play for grave offences. Penalty kicks are awarded for the more grave infringements such as foul play, but the player must indicate he is taking a shot at goal. A free kick is given for less serious offences and the beneficiaries cannot kick for goal.

RESTARTS
The ball is kicked from the halfway line to start the match off at the beginning of each half. Following a try the non-scoring side will restart the match with a kick.

SCRUMS
Restarts play after a minor infringement like a forward pass or a knock on. The forwards in each team bind together, push against the opposing team forwards in an effort to win the ball. The scrumhalf from the non-offending side feeds the ball into the scrum, normally from the left side of the scrum where his tighthead flanker is. The hooker which is the man in the middle of the front row will attempt to backheel the ball, as the scrum using its legs attempts to heel the ball back out to the scrumhalf.

LINEOUTS
Restarts play when the ball goes out of bounds (in to touch). The forwards line up in a parallel line perpendicular to the sideline and jump for the ball. The forwards cannot line up any closer than the five meter line from the touch-line.

OFFSIDE
One of the most confusing rules for spectators and often for players but an intrinsic part of the game.
A player is offside if he is in front of a team-mate who has the ball or who last played it and is interfering with play. Offside is penalized by a penalty kick where the offence takes place, or, in free play, there is the option of a scrum.
In static plays such as scrums, rucks and mauls, two different offside lines are created behind the last foot of the last player for each side.
An offside player may be made onside if a team-mate carries, kicks or pursues the ball past him.
If an offside player's position was unavoidable and he immediately moves back without interfering with an opponent, he will not be penalized.
Should the player be unsuccessful in avoiding contact and he is ruled "accidentally offside" a scrummage will be formed.

TACKLING
A player is tackled if he is brought to the ground by an opponent. He must then release the ball. Tackling around the neck is illegal.

FORWARD PASSES
A player must be either parallel or behind the team-mate who is passing him the ball. Players are also not allowed to knock the ball forward with hands or arms.

RUCKS
An informal scrum where the ball lying loose on the ground is raked back with the feet.

MAUL
Similar goal to a ruck but the ball is wrestled back with the arms and is not on the ground.

MARK
This occurs when a player behind his own 22 (including behind the try-line) cleanly catches the ball from a kick, knock-on or throw forward by one of his opponents and at the same time yells "Mark". The player does not need to have either foot on the ground to call a mark.
The free kick conceded must be taken by the player who made the mark, unless he was injured in doing so. If he cannot take the kick within a minute a scrum will be formed, and his team will put the ball in.

ADVANTAGE
The referee will not always stop play for an infringement, if after the offence the non-offending team has gained the advantage. The advantage must be either territorial or possession of the ball.
Advantage does not apply when: 1) The ball or the player carrying the ball touches the referee. 2) The ball emerges from either end of a scrum without being played. 3) A player is "accidentally offside".

For more information about the rules, visit the BBC Site