Okay, so in light of recent events here on MFG, I've decided to take it upon myself to write up a quick little thread on how to read a tire size.
We'll use the stock tire size on a MKIV 1.8T for reference. 225/45/17. Let's break this down.
225 - This means that the tire is 225mm wide from the outside of the tire wall all the way to the opposite tire wall. Please note: This does NOT apply to stretched tires (I can foresee those questions coming). This means when the tire is NOT mounted or the sidewalls are straight up and down when mounted, that will be how wide the tire is.
45 - This is the aspect ratio of the height to the width. It's basically saying that the height is 45% that of the width. 225 x 0.45 (or 45%) = 101.25mm. That's how TALL the tire is. The bottom of the sidewall (again, before being mounted) to the top of the tire (the tread).
17 - This is how big the tire is. What size wheel it will fit. Etc. This one is pretty easy.
There are other parts to reading a tire as well, but they shouldn't apply too much. Of course, there's the speed rating, but if you're trying to go fast you should already be well educated on tires and this write-up should have no bearing on you. Other small stuff to look for is a "P" before the series of numbers. This means it's a passenger tire vs. a truck tire. If you ever see something like 225/45/R17, that R simply means it's a radial tire which almost all of the common passenger car tires are. I don't remember seeing anything in a LONG while that has said differently.
If you'd like to know more, this is where I got a lot of this information (mostly because I couldn't figure out how to word anything without become too confusing) - http://www.ehow.com/how_2364482_read-tire-sizes.html
I hope this helps some people.
Edit: If I left anything that's deemed important out, please let me know. I'll be happy to edit it in or correct any mistakes, etc.