There are three types of indoor trainers: direct-drive smart trainers, wheel-on smart trainers, and wheel-on dumb trainers.
Direct-drive smart trainers have their own cassette and see your bike attached to the unit itself without the rear wheel. Inside they will have a resistance unit and a power meter that’s controlled by an app, so if your workout calls for 200 watts, it will make you pedal 200 watts. Most smart trainers are ANT+, ANT+ FEC and Bluetooth enabled and speak the same language as apps like Zwift, TrainerRoad, and The Sufferfest to provide interactive resistance.
Wheel-on smart trainers grab your bike by the rear axle and press a roller drum against the tire to create resistance. Similar to their direct-drive cousins, these trainers have electronics built-in, with the same connectivity designed to work with apps and create an interactive ride. Because your drivetrain isn’t directly connected to the trainer, the power readings aren’t quite as accurate; also the tire needs to warm up and you’ll need to complete a spin-down calibration.
Speaking of tires you’re also going to want a trainer tire like the Tacx Trainer Tire or Vittoria Zaffiro Pro Home Trainer Tire which uses a hard compound to prolong life and limit noise. They can be pretty hard to find, especially in MTB sizes, so a cheap slick will suffice. We’d also recommend buying the most inexpensive wheel you can find, that way you’re not trying to swap tires around before you hop on the trainer. If your bike uses a thru-axle, you’ll also need a trainer-specific one like the Tacx E-Thru-Axle, Kinetic Traxel, or a Robert Axle Project Trainer thru-axle.
Then, you have wheel-on dumb trainers, which use the same wheel drum-style resistance unit but don’t have any electronics. You can still get a solid workout with one of these, but you will have to find the right gearing and cadence to hit your targets. Because there are no electronics, these trainers are also quite inexpensive in comparison to their smart peers.