In basketball, the offense must shoot the ball before the clock counts down to zero. If they do not, they offense loses posession. In the NBA, the shot clock is 24 seconds long. In college and international competition, it is 45 seconds.

joev is correct on the NBA's 24-second shot clock length, but not so on college or international. In men's NCAA basketball, the shot clock is 35 seconds long; in women's, it is 30 seconds long. In international competition, men's or women's, the shot clock lasts 30 seconds.

The shot clock was devised by Danny Biasone, owner of the Syracuse Nationals of the NBA, after delay tactics had gotten so bad that on 22 November 1950, the Fort Wayne Pistons beat the Minneapolis Lakers by a final score of 19-18. The 24-second shot clock was implemented in the NBA for the 1954-55 season, and was an instant hit.