Exercise balls are large, plastic inflatable balls, usually a few feet in diameter. In the gym, these balls are used to teach balance and develop core muscles. When it comes to seating, an exercise ball helps users keep their backs straight and keeps them moving in order to maintain their balance -- not enough to be disruptive, but enough to keep the blood flowing in their legs and improve their posture.
Ergonomic stools resemble normal stools, but they are attached to flexible bases that can swivel in different directions. The combination of the seat platform and movability makes the ergonomic stool similar to an exercise ball, but without the inflation and spherical problems.
The kneeling chair has a seat platform like a normal chair, but no back: instead, the seat tilts forward, placing the user in a position where they would normally slide forward. The second part of the chair is a padded bar that extends in front of the user. When you sit in the chair, you prop your knees up to keep from sliding down. This puts your spine in proper alignment, but also puts stress on the shins and knees. This chair is also more difficult to get in and out of than other chairs.
A saddle chair is a high seating platform that is designed to support a user partway between standing and sitting. This allows the user to rest their weight against the chair, but also rise to their feet very easily -- the knees are bent very little, if at all. This decreases any fatigue associated with sitting for a long time and helps improve spine alignment.