We won't go into details about how wheels work, how they are important and what they are made of. We'll focus only on the relation between wheels and the esk8 world.
Quick overview of wheels theory
In general, bigger wheels means better comfort. Small rocks won't be an issue and you'll ride smoothly over cracks and bumps. Bigger wheels also means higher max speed.
Wheels have different hardness, measured on the durometer scale. Small number means the wheel is soft, high number means the wheel is hard (check this article for more info). Softer wheels are better for comfort (they absorb shocks better) and adherence. Harder wheels are better for high speeds and sliding.
Skate wheels and electric skateboards
Once again, this only applies to the belt and pulleys setup. Hub motors are out of this, since the hub motor is the wheel and you won't be able to choose what wheel you want to use.
There are dozens of different wheel brands and models for skateboards and longboards out there, but the kind of wheel required is very specific and in the end, there isn't much choice.
First, due to how the pulley is fixed, the wheel needs to have a centre hub with holes, to either put screws through it or allow the pulley clips to fit in (depending on the pulley you have). The pulley diameter is also an important factor, your wheel needs to be bigger than the pulley.
In the end, there are not a lot of compatible models left. The most used ones are the Orangatang Kegel (80mm) and the Abec11 Flywheel (76, 83, 90, 97 and 107mm). The Flywheels are quite expensive but e-riders often say they are simply the best. If you can afford it, you should probably get a set of the 90mm ones (or bigger). The Kegel are pretty good too, cheaper than the Flywheel but available only in one size. That's what the Boosted Board 2 is using.
Flywheel have been cloned a lot due to their friendly core for esk8s, with different quality depending on who makes them. But clones are never as good as the original one. If you've never used a longboard, cheap clones are probably a good option to start with. And as you get comfortable with your board and speed, you can eventually upgrade to the Abec11 ones and rich the nirvana.
If this kind of wheel isn't big enough for you, you can go for tire wheels. They are bigger, and of course the tire is doing a very good job at going over bumps and cracks. Those are not suited for all boards though, as you need a lot of clearance.