- Diameter, which means how tall the wheel is. This is measured in millimeters (mm) for skateboard wheels.
- Durometer, which means how hard the wheels is. Most skateboard wheels use what's called the "a-scale". You don't need to understand all the details of how the a-scale is measured, just that the higher the number, the harder the wheel. For example, you'll see the hardness of the wheels written "a95" for a an average street skateboarding wheel. Softer wheels can be all the way down to a70, or even lower.
Transition / Vert
Larger skateboard wheels roll a lot faster, and when riding ramps this is what you want. Try 55-65mm size wheels (though many ramp skateboarders will use even larger wheels -- try something like a 60mm wheel first, as you learn), with a hardness of 95-100a. Some wheel makers, like Bones, have special formulas that don't list durometer, like the Street Park Formula.
Street / Technical
Skateboarders who like doing flip tricks often like smaller wheels, as they are lighter and closer to the ground, making some skateboarding tricks easier and faster. Try 50-55mm skateboard wheels, with a hardness of 97-101a. Some brands, like Bones, make special Street Tech Formula wheels that also work very well, but don't have a hardness rating.
Both / All Terrain
You'll want something in the middle, with slightly softer skateboard wheels. Try a wheel size 52-60mm, with 95-100a hardness. This should give you a balance between speed and weight.
Usually cruising wheels are much larger for speed (64-75mm) and much softer for riding over rough terrain (78-85a). Other wheels for cruising are available, such as huge dirt wheels with knobs, but these aren't recommended for skateboards (try longboards or dirtboards).