There are four types of rowing machines: hydraulic, flywheel, water and magnetic.  Hydraulic machines tend to be the most affordable and the resistance is created from the amount of air or fluid that's constricted with a hydraulic cylinder. Flywheels work with fan blades to create resistance using air. This is the traditional rower that you might've seen at your local gym. Water and magnetic rowers are newer machines that are quieter and intended to mimic outdoor rowing the best. The difference is magnetic rowers use magnets, while water rowers use water in their tank to generate resistance.