A batsman is called out through a run out when the fielding team manages to break the wicket before the batsman has any part of his body or bat over the bowling crease. The fielder, who breaks the wicket, may either break it with the hand in which he has the ball or by throwing the ball at the wicket. However, if the batsman leaves the bowling crease to avoid getting injured by the ball while a fielder attempts to break the wicket, he cannot be called out.
Notice, not just the striker can be run out, but also the non-striker can be run out. For example if the striker hits the ball in direction of the non-strikers wicket and the bowler touches the ball before the wicket breaks and the non-striker is not behind the bowling crease, then the non-striker can be run out. However, if the ball breaks the wicket of the non-striker without somebody from the fielding team touching it, then the non-striker can not be called out.
Also if the non-striker leaves the bowling crease, to get ready to run to the opposite wicket, while the bowler attempts to bowl the ball, then the bowler may break the non-striker’s wicket.
The batsmen can not be run out while the ball is dead, so they may discus tactics in the middle of the pitch, while the fielding team is getting ready.