A lineout is awarded when the ball is carried or kicked over the touch-line, i.e., the ball has gone out of bounds. The team not responsible for the ball going out of play is awarded the line-out.
The lineout takes place in an area marked 5 meters from the touch line and 15 meters from the touch line.
The lineout usually consists of 7 players from each team standing a single file perpendicular to the touch line with a 1 meter gap between each team. The ball is thrown in from the touch line and must pass straight down between both lines of players who must compete for the ball by jumping.
Players not in the line-out must remain behind the off-sides line (10 meters back from the line-out) until the line-out is over. The exceptions are the thrower (usually #2) and their opposite number and both scrum-halves (#9).
Line-outs take place where the ball crosses the touch line. An exception to this is when a ball is kicked from outside the 22 meter line and does not land in play before crossing the touch line. In this case, the line-out occurs back in line from where the ball was kicked. The ball may be kicked directly into touch outside the 22 meter line only from a penalty.
Since the ball must be thrown in between the two teams, the only advantage that a team has is knowing where their own ball is going. Teams often have secret signals or calls for different types of lineouts.
Lineouts are commonly thrown to:
Here are a couple quick animations of throws to jumpers in the lineout.
Throwing to jumper one. The first and third players are lifting the first jumper, the fourth and sixth jumpers are lifting the second jumper to confuse the other team.
Throwing to jumper two. The first jumper is also being lifted, so the ball must be a lob which will pass out of the reach of the first jumper on BOTH teams.