It is a means of re-starting the game following a rule infringement.
The ball has been knocked on
The ball has gone forward
Accidental offside
The ball has not come out from a ruck or maul
Only eight players from each team can take part.
The hooker, two props, two second rows, two flankers and the number eight all bind together in a 3-4-1 formation, ready to lock heads with the opposition.
The hooker is the all important player in the scrum - it is their job to get the ball out for the scrum-half.
To make the hooker's life that bit easier are the two props.
They bind on tightly on both sides of the hooker, leaving no gaps between them.
Next comes the two second row forwards.
They bind tightly together and pack down behind the front row, putting their heads in the gaps between the hooker and the props.
Then come the two flankers, who bind on to both the props and second rows on either side of the scrum.
The last, but by no means least, of this exclusive gang is the number eight.
They pack down behind the second row forwards, putting their head inbetween the two second rows.
And there you have the scrum - one of the most spectacular sites in rugby union.
The scrum is formed at the place where the infringement happened.
All scrums must take place at least five metres from the touch or trylines.
Feeding the scrum: The team that was not responsible for the stoppage is awarded the ball.
The scrum-half is the player who gets things going in the scrum.
It's their job to feed the ball into the scrum for the hooker to strike back to the number eight.
The scrum-half must roll the ball in from the left-hand side, or the loosehead side as it is called.
From there, the scrum-half cannot handle the ball until it has come out of scrum.
The six other backs must be at least five metres behind the last forward of their team.
If they are not, the referee will penalise the offending team.
Hooking the ball
When it comes to scrums, the hooker is the player with all the responsibility and pressure.
Their job is to strike the ball back to the number eight once the scrum-half has feed the ball into the scrum.
But this is not as easy as it sounds.
Why? Because the opposition's hooker is trying to steal the ball from you.
Plus you've got eight huge forwards on the other side trying to push you off the ball.
The hooker is the only player in the scrum who can raise their feet - otherwise they would never be able to strike the ball.
However, no other player in the scrum is allowed to handle the ball until the ball is free - not even the hooker.
A scrum is finished once the ball has come out of the scrum.
Once it has, then the opposition scrum-half can tackle their opposite number for the ball.
But in some situations the number eight may dribble with the ball, keeping it in the scrum.
This means the opposing scrum-half cannot get their hands on the ball because it's still in the scrum.
This often happens when the team in possession have an attacking scrum near their opponent's tryline.
When is a scrum re-taken again?
The referee is the man in charge on the pitch and if he's not happy with a scrum, he can order it to be re-taken again when:
The scrum has rotated 90 degrees
The scrum has collapsed before the ball has been fed or before the ball has come out
The ball does not come out quickly enough.
The above is a bit simplistic, but in essence it is correct.