The reformer is a fundamental piece of equipment that adds resistance to Pilates routines. The reformer is used, for example, in power Pilates and Stott Pilates. It looks something like a crossbreed between a mousetrap, a NordicTrack, and a medieval stock, and is central to building core strength and flexibility.
There are many kinds of Pilates reformers - some are wooden and others are metallic and modern looking; some make use of leather straps, and others feature ropes. All reformers make use of a head rest, shoulder block, foot bar, and a carriage - the flat portion that moves forwards and back with your body.
The resistance is created by a set of springs, each coil which is color-coded based on how much force it generates. (If you're a novice, ask your trainer for a quick tutorial.)
The Pilates reformer can be used alone, but the addition of a metal or wooden attachment called a Pilates tower (made up of levers, springs, foot straps, and arm straps) can increase the resistance of each movement, making the workout more challenging. Some Pilates towers are also designed to be attached to a wall and used without a reformer.
Other names for the Pilates tower are the Pilates Cadillac and the trapeze table.