Magnetic turbo trainers
The most basic turbo trainers, these use magnetic resistance to imitate the feel of the road, and are generally the cheapest due to their simplicity. There are, however, few smart versions available. Usually supplied with a manual resistance changer, their smart capabilities aren't as varied as others and can't be programmed to replicate a certain gradient or power resistance.
Wheel-on trainers of any type tend to eat through tyre rubber quickly. You can buy a trainer tyre for your rear wheel which is made of more resistant rubber, but this then means that you need to have a second wheel available or make a tyre swap when you want to venture out onto the road.
Fluid trainers
A step up from the magnetic trainers, fluid models are much quieter and have a progressive resistance curve, meaning the faster you get, the harder it is to ride. These are where the majority of affordable smart trainers will be based as there is more room to integrate smart capabilities to fluid trainers, have a good ride and still hit a lower price point.
Direct drive trainers
Direct-drive turbo trainers take the bicycle's rear wheel out of the equation by attaching directly to the drivetrain, hence the name. As the drivetrain is directly linked to the turbo trainer, taking its power measurements from the cassette rather than the tyre, direct drive is the most accurate type.
These turbo trainers are usually electronic and have to be plugged into the wall so that their motors can be powered and resistance can be automatically altered when an app is controlling it.
By forgoing the need for the rear wheel, you don't have to buy new tyres regularly, as you would with any of the wheel-on trainers.