A balk is an illegal motion made by the pitcher that can take place when there is one or more runners on base. While a number of moves may result in a balk, the typical cause is a desire by the pitcher to catch runners off balance. Most of the time, the pitcher is pretending to pitch when he has no intention of doing so.
A pitcher is restricted to certain motions and pitching positions before and during a pitch. If a pitcher violates any of these motions while runners are on base, the umpire may call a balk.
Examples of a balk include:
– a pretended throw to first or third base or to the batter with one foot on the pitcher’s rubber
– a pitch in which there is either an insufficient or too long a pause after the windup or stretch
– dropping the ball while on the rubber, even if by accident, if the ball does not subsequently cross a foul line
– while on the rubber, making a motion associated with the pitch and not completing the delivery
For a more complete look at what constitutes a balk, you can refer to the Official Rules here.
The penalty for a balk is an advance of the runner(s), with each runner being awarded the next base.