Pilates Reformer is a bed-like frame with a flat platform on it which rolls back and forth on wheels within the frame. This platform is called a carriage and it is attached to one end of the reformer by a set of springs. The springs provide choices of differing levels of resistance as the carriage is pushed or pulled along the frame.
The reformer also has long straps with handles on them that are attached to the top end of the frame. They can be pulled with legs or arms to move the carriage as well.
Body weight and resistance of the springs are what make the carriage more or less difficult to move. The Pilates Reformers parts are adjustable for differing body sizes and for differing levels of skill, providing individual flexibility.