Well, I'm going to take my time answering this....
A ruck is a phase of play where one or more players from each team,
who are on their feet, in physical contact, close around the ball on the
ground. Open play has ended.
Rucking. Players are rucking when they are in a ruck and using their
feet to try to win or keep possession of the ball, without being guilty of
foul play.
16.1 FORMING A RUCK
(a) Where can a ruck take place. A ruck can take place only in
the field of play.
(B*) How can a ruck form. Players are on their feet. At least one
player must be in physical contact with an opponent. The ball is
on the ground.
16.2 JOINING A RUCK
(a) All players forming, joining or taking part in a ruck must have
their heads and shoulders no lower than their hips
Penalty: Free Kick
(B*) A player joining a ruck must bind onto the ruck with at least one
arm around the body of a team mate, using the whole arm.
Penalty: Penalty Kick
(C*) Placing a hand on another player in the ruck does not constitute
binding.
Penalty: Penalty Kick
(d) All players forming, joining or taking part in a ruck must be on
their feet.
Penalty: Penalty Kick
16.3 RUCKING
(a) Players in a ruck must endeavour to stay on their feet.
Penalty: Penalty Kick
(B*) A player must not intentionally fall or kneel in a ruck. This is
dangerous play.
Penalty: Penalty Kick
(C*) A player must not intentionally collapse a ruck. This is dangerous
play.
Penalty: Penalty Kick
(d) A player must not jump on top of a ruck.
Penalty: Penalty Kick
(e) Players must have their heads and shoulders no lower than their
hips.
Penalty: Free Kick
(f) A player rucking for the ball must not ruck players on the ground.
A player rucking for the ball tries to step over players on the
ground and must not intentionally step on them. A player rucking
must do so near the ball.
Penalty: Penalty Kick for dangerous play
16.4 OTHER RUCK OFFENCES
(a) Players must not return the ball into a ruck.
Penalty: Free Kick
(B*) Players must not handle the ball in a ruck.
Penalty: Penalty Kick
(c*) Players must not pick up the ball in a ruck with their legs.
Penalty: Penalty Kick
(d) Players on the ground in or near the ruck must try to move away
from the ball. These players must not interfere with the ball in the
ruck or as it comes out of the ruck.
Penalty: Penalty Kick
(e) A player must not fall on or over a ball as it is coming out of a ruck.
Penalty: Penalty Kick
(f) A player must not take any action to make the opposing team
think that the ball is out of the ruck while it is still in the ruck.
Penalty: Free Kick
16.5 OFFSIDE AT THE RUCK
(a) The offside line. There are two offside lines parallel to the goal
lines, one for each team. Each offside line runs through the
hindmost foot of the hindmost player in the ruck. If the hindmost
foot of the hindmost player is on or behind the goal line, the
offside line for the defending team is the goal line.
(B*) Players must either join a ruck, or retire behind the offside line
immediately. If a player loiters at the side of a ruck, the player is
offside.
Penalty: Penalty Kick
(C*) Players joining or rejoining the ruck. All players joining a
ruck must do so from behind the foot of the hindmost team mate
in the ruck. A player may join alongside this hindmost player. If
the player joins the ruck from the opponents’ side, or in front of
the hindmost team mate, the player is offside.
Penalty: Penalty Kick on the offending team’s offside line
(d) Players not joining the ruck. If a player is in front of the
offside line and does not join the ruck, the player must retire
behind the offside line at once. If a player who is behind the
offside line oversteps it and does not join the ruck the player is
offside.
Penalty: Penalty Kick on the offending team’s offside line
16.6 SUCCESSFUL END TO A RUCK
A ruck ends successfully when the ball leaves the ruck, or when the
ball is on or over the goal line.
16.7 UNSUCCESSFUL END TO A RUCK
(a) A ruck ends unsuccessfully when the ball becomes unplayable and
a scrum is ordered.
The team that was moving forward immediately before the ball
became unplayable in the ruck throws in the ball.
If neither team was moving forward, or if the referee cannot decide
which team was moving forward before the ball became
unplayable in the ruck, the team that was moving forward before
the ruck began throws in the ball.
If neither team was moving forward, then the attacking team
throws in the ball.
(B*) Before the referee blows the whistle for a scrum, the referee allows
a reasonable amount of time for the ball to emerge, especially if
either team is moving forward. If the ruck stops moving, or if the
referee decides that the ball will probably not emerge within a
reasonable time, the referee must order a scrum.
I actually typed all those.. I got it from the law book though.. but it is pretty simple, ruck has plenty of laws written for it for such a simple thing...