The reformer was invented by Pilates founder Joseph Pilates in the 1940s. It is a piece of equipment with a bed like frame with a flat cushioned platform called the carriage, which rolls back and forth on wheels within the frame.
The carriage is attached to one end by a set of springs, with different resistances. At the other end are long straps, which provide opportunity for upper body/arm or lower body/leg exercises. This makes for a versatile and very flexible piece of equipment.