Magnetic bike trainers use a magnetic resistance system to create resistance when pedaling. The magnets are positioned around the wheel and as you pedal, they move closer together or further apart depending on the level of resistance desired. This type of trainer is typically quieter than other types, making it ideal for indoor cycling sessions.
Fluid bike trainers use a fluid-filled chamber to create resistance when pedaling. As you pedal, the fluid inside moves against a flywheel creating drag which increases with your speed and effort output. Fluid trainers tend to be more realistic in terms of road feel compared to magnetic trainers and also provide progressive levels of resistance that increase as you ride faster or harder.