Paly will only be stopped wen the ball is loose if there is no advantage. This normally happens if a team knocks the ball forward from their hands and if he other team does not benefit then they are called back for a set piece in favor of the team that did not make the mistake. A ruck is when 2 players from one team or more and at least one from the opposing team ( has to be 3 or more players) are on their feet fighting for the ball. You cannot use your hands and either push people out the way so your body passes the ball so it is now on your side and is now legal to pick up. These are the real basics along with a forward pass which now means a scrum will happen. The ball can only travel forwards from a kick and not the hands. You will pick this up slowly the more you watch.