Water: They use water in a sealed tank. They offer a more realistic feel of actual rowing on a lake/sea, accompanied by the relaxing sound of water.
Magnetic: They use magnets to adjust resistance. Ideal for apartments, as they make no noise at all and allow you to work out at any time.
Air: The faster you pull, the more resistance they create. They offer a more dynamic and demanding workout for CrossFit/HIIT, but they produce noticeable noise (like a fan).
Hydraulic: They operate with hydraulic shock absorbers on the arms. They are the most economical option, take up the least space, but have a more limited/mechanical motion.