A ruck is a phase in play which takes place after a tackle. The rule in Rugby Union is that a player taken off his feet must release the ball "immediately". That is one of the fundamental rules in rugby -- a player who is off their feet can't play the ball. However a tackled player is allowed a second or two to plant the ball on the ground, hopefully on his own side of the field, where his body (to a degree) shields the ball from opposing players. Once the ball is planted, both sides are free to compete for the ball. If someone is quick enough they can just pick it up and go, but generally this doesn't happen. There simply isn't time before the supporting players arrive. Instead the ball gets caught up underneath a number of players from each side who run in and then try to push their opponents back and off the ball. At this point -- where the ball is between or under opposing players who are bound to and shoving away at each other -- that the referee will shout "it a ruck". Once a ruck is formed, players in the ruck can no longer play the ball with their hands. Instead they try to shove their opponents off the ball while raking it back to their feet, until another player (not bound to the ruck) can play the ball from the behind them. Because the tackled player can plant the ball to his own side of the field the attacking side has an advantage, but it is only an advantage. Every time the ball goes into contact, the defending side has a reasonable shot at turning it over.
A ruck is formed when a duck mates with a rock. Typically, the rock need not be tackled first; usually it just lays there.
