Durometer is the number followed by an A on every wheel. Higher means harder.
Hard wheels (95A to 101A)
Fast and slidey on smooth concrete. Ideal for skateparks, ledges and technical street.
Soft wheels (78A to 90A)
Grip and roll over cracks and pebbles. Perfect for cruising, rough streets and getting from A to B.
In between
Hybrid wheels around 92A are a great compromise if you skate a bit of everything.