generally summoned when an error of play occurs according to the law book. E.g. foward pass, knock on/ lost forward, free kick and any other play that is an infringement but fall short of the straight arm penalty.
ruck and maul are the same with one difference
ruck is when the ball goes to ground.A tackled player goes to the deck and at any stage where ball makes contact with the ground, that is a ruck. in which hands aren't allowed. So moving the ball from a ruck to the back of the ruck with hands is a straight arm penalty or free kick, depending on the seriousness of the offence
maul on the other hand is when the ball of the tackled player never makes contact with the ground. Hence, when the ball is up above the ground, it is deemed a maul. Moving the ball with hands in the maul is allowed. There are variations between the two where ruck's definition crosses into the mall definition and vice versa in which the fine line between the two is indistinguishable.
E.g. Tackled player goes to ground but the ball never actually reaches ground because the ball is stuck between players, meaning the ball is still above ground, therefore what is supposedly a ruck is now a maul. Its up to the referee's interpretation of what is deem to be a ruck or maul.
The best way to probably remember them separately is that one can move and the other can't. Maul can move, whereas the ruck can't. Maul, you can use your hands, ruck, no hands.