I'm glad that you asked this question and Jetrinka did a great job of providing the answer.
The important thing to remember is choosing the right speed rating for you.
Speed ratings are primarily an indicator of how well a tire dissipates heat across the carcass of the tire. Heat destroys tires from the inside out.
However, if you do not or your vehicle is not capable of speeds in excess of XXX speed, then do you really NEED the Z rated uber tire?
Chances are you do not.
So, save a few bucks and get the best tire for your needs.
FORMER Avenging Moderator of the Apocalypse and Leader of the Axis of Oversteer
"I live with fear every day and sometimes, she lets me race."
2002 SVT Focus... Stock