I get a lot of PM's regarding this so I figred I'd make a sticky...
For example a 195/50/15 tire is 195mm wide, the sidewall is 50% of the width, and it's for a 15" rim
Here's another 275/35/22 is 275mm wide with the sidewall being 35% of that and for a 22" rim.
Here's a great tire size calculator I use that lets you compare tire sizes. http://www.1010tires.com/tiresizecalculator.asp
Here's a link to WAY more info about tire sizing then you'll ever need.http://www.discounttire.com/dtcs/infoSidewall.do