Getting run out in a match can be considered as a criminal waste of a wicket. But in spite of it, there are run outs galore that occur in every tournament. A batsman is ruled run out when in an attempt to take a run he finds himself short of his crease, at the time when the stumps are broken with the ball, either by the wicket-keeper or any other fielder.
Run out basically occurs when you have a misunderstanding between the two runners or when one has misjudged a run and finds oneself inches short of the crease. Run outs can happen at both ends of the wicket.
In the earlier days when the umpire had to give a decision with his bare eyes, the tendency of giving the batsman the benefit of the doubt was the norm but with the advent of the modern day technology all that has changed. In the earlier days if the batsman was an inch or a centimetre out of his crease he on most occasions would have got the benefit of the doubt but not these days.
The third umpire makes certain that he looks at the replay from all available angles and gives the right verdict.
When an appeal for a 'run out' is made and if the decision is a tight one then the field umpire may signal to the third umpire for a verdict. If the red light flashes then the batsman has been ruled out but if a green light flashes then the batsman is not out.