An off break is bowled by holding the cricket ball in the palm of the hand with the seam running across under all the fingers. As the ball is released, the fingers roll down the right side of the ball (for a right-handed bowler), giving the ball a clockwise spin as seen from behind. When the ball bounces on the pitch, the spin causes it to deviate towards the right from the bowler's perspective; this is to the left from the batsman's point of view, or towards the leg side of a right-handed batsman. The ball spins away from the off side, and this is where it gets the name off break, meaning it breaks away from the off.
An off spin bowler will bowl mostly off breaks, varying them by adjusting the line and length of the deliveries.
Off breaks are considered to be one of the easier spin deliveries for a right-handed batsman to play. This is because the ball moves in towards the batsman's body, meaning the batsman's legs are usually in the path of the ball if it misses the bat or takes an edge. This makes it difficult for the bowler to get the batsman out bowled or caught from an off break, but it does mean there is a chance of leg before wicket, assuming the ball has not turned enough to miss the leg stump.
A left-handed batsman has more difficulty facing off break bowling, because the ball moves away from his body. This means that any miscalculation can more easily result in an outside edge off the bat and a catch going to the wicket-keeper or slips fielders.
A ball bowled by a left-arm orthodox spin bowler with an off break action spins in the opposite direction. Such a ball is not normally called an off break, but a left-arm orthodox spinner.