Featuring running, passing, kicking and tackling, rugby is a 150-year-old game of football with fairly simple rules but very strenuous physical demands. The object is to score a try by carrying the ball over the goal line and touching it down. This is worth five points, and the conversion (which is not always attempted from right in front of the goal posts) is worth two. A team may also score by means of a penalty kick (three points) or a drop kick (also three points).
Any one of a team's 15 players may run with the ball at any time. Players may also kick it or pass it to any of their teammates. The pass must be lateral or backwards. If a player is tackled to the ground while carrying the ball, he must release it. (In rugby, tackling still means wrapping up the ballcarrier's legs — knocking a player off his feet by diving at him doesn't count, and is in fact illegal.)
Play does not stop after a tackle, but continues until someone scores, the ball goes out of bounds or an infraction or penalty occurs.
When a player is tackled to the ground, a ruck forms and the two teams try to push each other away from the ball, something like linemen trying to move one another out of the way in American football. If a player is held up but not tackled to the ground while in possession of the ball, then a maul forms, and the two teams try to wrest it away from one another by grappling with their arms and upper bodies. Often but not always the team that began with possession has an advantage in these situations.
If the ball goes out of bounds, then a lineout is formed. The tallest members of each team take up positions opposite one another, something like a line of scrimmage, and the team that didn't carry or kick it over the sideline throws the ball back into play. This offers the advantage of knowing where and when the throw will go; however, unless the thrower and the jumper get their timing right, the in-bounds play turns into a kind of jump ball.
When a team has committed an infraction, such as passing the ball forward or even fumbling the ball forward (called a knock-on, a frequent error similar to basketball's walking), eight players from each team join into a familiar, interlocking formation called a scrummage. The team that didn't commit the infraction rolls the ball into the middle of the scrummage and almost always wins possession of the ball.
Teams are penalized for more serious offenses such as offsides (a very tricky rule that generally means playing in front of the ball or ballcarrier) or foul play, such as a high or dangerous tackle. If the penalty occurs close enough to the goal posts, teams try to kick penalty goals; otherwise, they often kick it downfield and out of bounds to gain territory. Because of the penalty situation, the non-offending team also receives the advantage of the throw-in.
A rugby team is divided into forwards and backs. Forwards, usually larger, more powerful men, are chiefly responsible for winning the ball at mauls and rucks, and lineouts and scrummages. They also carry the ball in heavy traffic, such as fullbacks do. Backs are faster and better passers and kickers. When the forwards have won possession, they run set plays designed to fool the defense and spring the intended ballcarrier into the open field.
Not every play is designed to score; many are meant to gain territory or force the defense out of its alignment, while the offense rewins possession and launches another play. If the attacking team doesn't retain possession, however, the play goes down as a turnover, and the other team immediately begins attacking. Some plays, like a very long punt, intentionally turn the ball over in exchange for territory.
The game's great challenge is to execute sophisticated skills amid a steady pace of running and tackling. Players typically run about two miles over two 40 minute halves, separated by a five-minute intermission. No substitutions are allowed, except for injury. Like basketball or hockey, fewer stoppages indicate a higher level of play. Well-played matches typically see a total of three or more tries and scores just a little bit higher than pro football games.
SOURCE: USA Rugby