A "no ball" is a penalty imposed on a bowler for over stepping the crease. The umpire signals a no ball when the bowler crosses the bowling crease with his front foot. The bowler, however, can step on the crease.
Apart from conceding a run, the bowler will also have to bowl an additional run. The runs scored of a no ball are also valid. For example: If a batsman scores two runs of a no ball, then three runs are added to the total.
No balls are generally conceded by fast bowlers, who tend to over step when they try to go flat out.
A no ball is also signalled (by the leg umpire) when a bowler exceeds his quota of bouncers per over. The leg umpire will have to judge if the ball is proceeding above the shoulder of the batsmen before ruling it as a no ball.