Tire Size Definitions

What do the numbers mean?
Okay, I'm making this up as I go along, but here are what I believe tire numbers mean. For the most part, I'm pretty sure I'm right.

A. Wheel Rim Diameter - The outer diameter in inches of the wheel that the tire is designed to fit.
B. Tire Type - "R" stands for Radial. I believe all modern tires are radial, but in the old days they were bias-ply.
C. Tire Speed Rating - This letter corresponds to what speed the tire is designed to withstand under optimal weather conditions.
D. Aspect Ratio - The width of the sidewall, it is a percentage of the Tire Width.
E. Tire Width - The overall width of the tire in millimeters (mm).

Figuring Tire Dimensions
Using these numbers, you can easily figure the following tire dimensions:

Tire Width (w) - this is clearly marked on the tire. On the above example, the tire is 225mm wide.
Sidewall Height - Take the Tire Width as a percentage, and multiply it by the Aspect Ratio. On the above example, that would be 225mm * 60%, or 225 * 0.60 = 135mm. So the sidewall height is 135mm.
Overall Diameter (d) - Take the wheel rim diameter, and add it to 2 * Sidewall Height. For example, 16" + (2*135mm). Of course, we have to convert either the inches to millimeters or vice versa to do the math. 2*135mm comes out to be about 10.6". Add that to the 16" wheel, and we get a total of 26.6" as the overall diameter.

Common Tire Talk
When people say "40-series" or "50-series", they're simply talking about the Aspect Ratio of a tire. For example, a 245/40ZR18 tire and a 255/40ZR18 tire would both be "40-series" tires.
When someone calls a tire "Z-rated" or "V-rated", they're talking about the Tire Speed Rating.
When someone is talking about "245-wide" tires, they're talking about any tire whose overall width is 245mm.