Those who don't carry much fat around the middle may see the six distinct sections of the rectus abdominals, the long sheet of muscle that extends from your low belly to the area just below your sternum. All abdominal exercises work the whole muscle--you can't truly isolate "upper abs" or "lower abs." That said, some deeper muscle fibers will get more of a workout with certain types of abdominal exercises, such as those done on a stability ball.
The six sections of the rectus abdominals are not visible on most people, even those with low body fat, so it is more important to focus on strength than on whether you can see these sections. Don't get discouraged if your abs don't have that washboard appearance. Instead, remember that if you do abdominal exercises, the rewards range from better posture to a reduced risk of low back pain to stronger strength training workouts. You can strengthen your rectus abdominals using an abdominal machine if your gym has one, but some version of crunches on the floor or on a stability ball work just as well.
Other contributors to the appearance of six-pack abs are the internal and external oblique muscles that extend down the sides of the rectus abdominals. Keep in mind that simple twisting crunches, either on the floor or on a stability ball, will strengthen these muscles. There is no need to do side bends while holding weights or to do twists while holding a pole on your shoulders. Both of these moves can strain the lower back rather than strengthen it.