Traction, Tread Wear, and Temperature
Like being in school, there are grades for above-average, average, and below-average performance. Except in tire talk, there isn't a below-average score, as it would be simply too dangerous.
If a tire receives an A for traction, it means it's exceptionally well crafted for braking in wet conditions, but cornering isn't taken into account. If a tire receives an A for temperature, it means it's exceptionally well crafted at dissipating heat and not generating an excessive amount of it. For tread wear, numbers are given out. A number of 150 would be able to wear one-and-a-half times as a tire graded at 100, and so on.
Speed Ratings
Vehicles are going faster every year, and this includes SUVs and pickup trucks, despite their obvious limitations. It's important to know if you need a speed-rated tire or not, and it's important to know which tires are high-speed rated and which aren't. Speed ratings were first developed in Europe, where the grades went from A5 (for 15-mph forklifts) to Z for speeds above 149 mph.
The United States began grading tires in much the same way, but also stipulated that every tire must be capable of at least 85 mph. In a way, a speed rating is much like a movie rating. The higher on the alphabet, the more adult performance you can expect from it. (Except for H--someone messed up with that one.) Here's a list of the most common speed ratings.
S = 112 mph
T = 118 mph
H = 130 mph
V = 149 mph
W = 168 mph
Z = over 149 mph