A shot clock is used in some sports to quicken the pace of the game. It is normally associated with basketball, but has also found use in sports such as Snooker, professional lacrosse, professional Australian rules football and korfball.
In the NBA the shot clock is 24 seconds.
In basketball, the shot clock is a timer designed to increase the pace (and consequently, the score) in games. The offensive team must attempt a field goal before the shot clock expires, and the ball must then either touch the rim or enter the basket, or the offensive team will be assessed a violation resulting in loss of possession.