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.

Wheel sizes

52-55mm
Smaller Riders 

56-60mm
Beginners, all ramp, park and street

60mm+
Longboards, old skool decks, high speed and rougher surfaces