Magnetic and fluid bike trainers can look the same as the bike trainer stand looks the same. Once you get them up to speed, the difference is apparent.
Magnetic bike trainers use a magnetic flywheel to provide resistance. The flywheel has a set of magnets that create resistance as they’re engaged. This lets you choose the amount of resistance by rotating a dial on the wheelfly hub. Some options let you do this via a cable that mounts on your bike’s handlebar.
Fluid trainers have an inner chamber that is filled with fluid that thickens as the flywheel picks up speed. This provides a smoother power increase as you ride and is a closer simulation to road riding. Fluid trainers are generally quieter than magnetic trainers as well.
Fluid trainers tend to cost more than magnetic bike trainers for the reasons listed above. When choosing a wheel-on trainer, weigh the options between your priorities and the cost.
If closer simulation of road riding is your priority, the price could be worth it. If you want to put in miles or ride in your garage, the cheaper option might be ideal.