In golf, a flat backswing is one that runs closer to parallel to the ground, while a vertical, or upright, swing is one that runs closer to perpendicular to the ground. Legendary golfer Ben Hogan was famous for his flat backswing. Current PGA player Chad Campbell also has a noticeably flat backswing. On the other hand, Jack Nicklaus, John Daly and Jim Furyk use very upright swings. While not generally not taught to beginners, a flat backswing may have a few advantages.
Perhaps the most common problem among amateur golfers is slicing the golf ball, or hitting shots that curve drastically from left to right. A slice usually results from an outside-in swing plane, or one in which the club is taken back away from the body during the backswing and brought in closer during the downswing. This motion causes the club to cut across the golf ball, resulting in the left-to-right spin. With a flat backswing, it is nearly impossible to take the club on an outside-in path, and you may be able to reduce slicing with this style.
An upright swing will cause you to bring the club toward the ball with a steeper angle of attack. This steep downward compression on the golf ball will cause the ball to soar. A flatter swing generally will result in less downward compression and lower shots. Low shots can be advantageous in windy conditions. Often you will see professional golfers use a flatter swing when they try to keep the ball low.
One reason the ball soars higher from an upright swing is because this style places increased spin on the ball. A high amount of backspin generally is advantageous around the greens, when you want the ball to stop quickly. With a driver, however, you can greatly increase distance with more roll. Too much backspin will decrease the amount of roll you get with this club.
An upright backswing requires a higher degree of shoulder and back flexibility and is perhaps a more unfamiliar technique for the beginning golfer. A flat swing is more akin to swinging a baseball bat -- a much more common movement for many people. Because of its simplicity and lower demand for flexibility, a flat swing plane remains much easier to achieve as golfers age.