If the ball is held up off the ground, once more than any two players have bound together a maul is formed. If the ball has gone to ground, then the group of bound players is called a ruck.
The very important principle of rucks and mauls is that once they are set, two imaginary offsides lines become present at the back of each team's rucking/mauling players extending from touchline to touchline. Any player running into the zone who is not joining the ruck or maul, from behind this line, before the ball leaves is considered offside and a penalty can be awarded to the other team.