They are made up of two parts, the gum and the core (hub). The wheel is the important organ of the skate, it gives speed, responsiveness, grip and maneuverability.
For Derby rollerskating, if you want speed choose hard wheels (+ 88a) with a larger diameter (+ 60mm).
So everyone is talking to you about hybrid wheels? Of course !
Basically, it's the kind of wheels that go everywhere, medium hardness, they will go both outdoors (outdoor) and indoors (indoor), then it's up to you to choose them in large diameter or not and wide or not depending on your level skating and the usefulness you will have of it: blocker, jammer, pivot, leisure...
For recreationnal (hiking, strolling...) you will generally find rather soft wheels (78a to 84a).
Here too the diameter will give you rolling comfort and speed. The bigger you take, the better it will roll and the less you will feel the relief of the ground.
The width will affect your stability and your maneuverability, generally for leisure, we take fairly thin wheels in order to have maximum maneuverability (38mm and less).
In park/freestyle: in order to do tricks you have to use hard wheels, indeed during slides or skids, the wheels can rub on the modules... if they are too soft you will take "stops": the wheel at contact with the support will stop you dead and you will fall... with hard wheels it will continue to slide even if it will slow down the slide or skid a little.
Level width we are rather on the narrow to keep a maximum of maneuverability but also of gap between the wheels level of the trucks to slide above (in 50-50).
For the diameter as usual: the bigger it is, the faster it will go; the smaller it is, the more stable it will be...
TIP: freestyle skateboard wheels are perfect for bowl/park, if you go on this type of wheel, choose 55mm to 60mm in 101a.