In strict Rubgy (Union) terminology, a "ruck" or "loose scrum" is a contest for a ball which is on the ground in loose play, where players are bound on to each other. The players are supposed to get the ball back out using their feet only. (This differs from the set-piece scrum which is awarded after an infringement, in that a "ruck" usually forms when a player is tackled and brought to ground).

A loose scrum in which the ball is not actually on the ground is a "maul". This usually occurs where a player is tackled and held but not brought down. In this case, he will try to turn round and pass the ball backwards hand-to-hand; his own forwards will try and aid him while the opposition will try and snatch the ball away.

Off the Rugby field, "ruck" may will mean much the same as "brawl", but as with most posters here, I have heard it used all over Britain to mean "disorganised pack of people".