Air rowing machines
These use a flywheel to generate air resistance as you pull the handle. The resistance level can normally be set by you, but it's also determined by how fast you row - the faster you go, the greater the resistance.
Magnetic rowing machines
Magnetic rowers also use a flywheel, but resistance is generated by a magnetic braking mechanism within it, making them a lot quieter. When you set the resistance level it won't change with your rowing speed.