Usually, after fielders collect the ball, they throw it towards the fielders at either end of the wicket. The ball is officially dead when the receiving fielder collects the ball. However, in cases where the fielder throws a completely misdirected ball or if the receiving fielder misfields in collecting the ball, then the ball is considered still in play. The batters can then add more runs by running between the wickets. This act of the fielding side is called an overthrow.
If a batter hits a boundary, then the ball becomes dead upon making contact or crossing the boundary line. Now consider a scenario where the ball is collected by the fielder and an overthrow ensues. If the overthrown ball reaches the boundary, then the batting side gets four runs in addition to the number of runs they ran between the wickets.
The next scenario is when an overthrown ball does not reach the boundary and is collected by a fielder. If this fielder then commits an additional overthrow then the ball is still in play and the batters can run till the ball is dead. This means, in theory, an unending overthrow can result in unlimited runs scored of a single ball.
Another scenario is when the batter dives to reach the popping crease but the ball hits the bat of the batter unintentionally. In such a case, the deflection caused by the bat is legal and the ball is in play. The batters can continue to run between the wickets.
Finally, consider a scenario if a fielder hits the stumps, but the batter is well inside the crease. Should the ball deflect after the stumps are hit, then the ball is considered in play. An overthrow is identified and batters run to add to their scorecard.
An overthrow is a frustrating moment for the fielding side. The maximum runs scored of an overthrow in international cricket are eight runs. In this case, the batters ran four runs. Later, an overthrown ball reached the boundary, totalling the number of runs on that delivery to eight.
Overthrown are also called buzzers in cricket. Two theories, though not definite can explain this terminology. The first theory suggests that buzzers are referred to anything eccentric that is flying around. Since an overthrow is a wild and eccentric, it might have acquired the term.
A more plausible theory explains that a bullet that missed its target in the first World War was called a buzzer. Since overthrow occurs as a result of the ball missing the stumps, it is logical to tag it as a buzzer.