There is a lot of confusion about tires from the side wall markings to performance tradeoffs. The Tire Rack has an extensive library of good basic information; some of it is a little over simplified but still a good resource.

Side wall:
http://www.tirerack.com/tires/tirete....jsp?techid=33

Speed ratings:
http://www.tirerack.com/tires/tirete....jsp?techid=35

Their library:
http://www.tirerack.com/tires/tiretech/tiretech.jsp