Hard wheels: Fast
Soft wheels: Slower, but more grip
87a - Soft wheel for rough surfaces, great grip for longboards and cruisers.
95a - Slightly harder and faster, medium grip
99a - Happy medium between speed and grip, solid all around wheel
100a+ - Hard and fast wheel, low grip.
Beginners, all ramp, park and street
Longboards, old skool decks, high speed and rougher surfaces