Invented by Pilates founder Joseph Pilates, the Reformer is a bed-like frame with a flat platform on it, called the carriage, which rolls back and forth on wheels within the frame.
The carriage 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 carriage has shoulder blocks on it that keep a practitioner from sliding off the end of the reformer as they push or pull the carriage.
At the spring end of the Reformer there is an adjustable bar called a footbar. The footbar can be used by the feet or hands as a practitioner moves the carriage. 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. Reformers parts are adjustable for differing body sizes and for differing levels of skill.