SPEED is the term used to represent the wider profile wheel.  This wider shape provides more grip when cornering; especially at high speeds.  These wheels are used by the recreational session skater, jam skater, roller derby skater or the competitive speed skater.
 
Wheels are measured in 'mm' which represents the circumference of the wheel.  Taller (62mm) wheels are for recreational skating, jam skating, roller derby skating or competitive speed skating.

The hardness of the wheel will determine the amount of control you have.  A softer wheel (90a-98a) will "grip" the skating surface and allow for more control.  A harder wheel (101a-103a) will "slip" and give a more uncontrolled roll.