Wheel durometer is how wheel hardness is measured. The higher the number, the harder the wheel.
Most commonly used is the durometer “A” scale. A very hard wheel would be around 101a and a very soft wheel around 78a.
Hard wheels roll best on smooth ground and slide easily.
Soft wheels roll fast on rougher surfaces as well as smooth. Soft wheels absorb bumps, cracks and pebbles better than hard wheels but don’t slide as easily.
Hard wheels like 101a-97a are ideal for skateparks, ramps, and smooth flatground.
Soft wheels like 78a-88a are great for transportation, rough skate spots and are the wheels used for cruisers and longboards.
Wheels in the 90a-95a range are typically hybrid wheels and have some of the characteristics of hard and soft wheels. For example, a 92a will work on ground that isn’t perfectly smooth and still slide without too much effort.