Chair is a piece of equipment that you use in Pilates in order to perform different exercises on. The most common machines and equipments in Pilates are Chair, Cadillac, Reformer and Barrel. They all look very different, but you can perform the same exercises on them. The fact that they are designed differently, will allow you to select the type of equipment best suited for your range of movement, and your goals with the workout.
For instance, a Swan Dive exercise performed on the Chair compared to the Reformer, may be easier or harder for you depending on your body. You may find it easier to engage the target muscles better on the Chair compared to the Reformer, or feel that you challenge certain body parts more on the Chair compared to the Reformer or Barrel.