A no-ball is a delivery which does not count as one of the bowler's six legitimate balls in one over.
The fielding team are penalised one run every time a no-ball is bowled, which is added to the extras tally of the batting team.
The extra run will also be added to the bowler's overall figures.
The umpire will call a no ball by raising an arm at shoulder height and the fielding team must bowl another legitimate delivery.
If the batsman scores off a no ball, the runs will be added to their individual score.
There are a couple of anomolies, though.
In domestic 40-over cricket, a no-ball concedes two runs. In Twenty20 cricket, a no-ball is followed by a 'free-hit', a delivery from which the batsman can not be bowled or caught out, but can still be run out.
They cannot be dismissed off a no ball - only except if the batsman is run-out.
The umpire will call a no ball if:
The heel of the bowler's front foot lands on or in front of the popping crease (the front line of the batting crease).
However the front foot can be raised over the line as long as the heel does not go beyond the popping crease
The bowler's back foot is touching or outside the return crease
A full toss - a ball which does not bounce - from a seam bowler reaches the batsman at waist height.
However, a waist-high full toss is permissable from a slower bowler, as long as it doesn't go above the batsman's shoulder. If it does, then the umpire will call a no-ball
The bowler does not notifies the umpire of a change in their mode of delivery. So if a player says they are a right-arm bowler to the umpire and then bowls left-arm, the umpire will call a no ball.
The umpire believes the bowler is throwing the ball
The bowler bowls the ball before entering their delivery stride
If the ball bounces more than twice before it reaches the batsman
If the ball rolls all along the ground
The ball stops in front of the batsman without having touched the bat
If the wicket-keeper encroaches beyond the stumps before the ball has been struck by the batsman or has passed the stumps
More than two fielders are positioned on the leg side behind square
The umpire deems the bowler to be bowling dangerously and unfairly