Help with Rims/Tires numbers?

[John99Boxster]

I have a basic understanding of tires and the numerical designations assigned to them. (i.e. 255/40 17 -- 255mm width of tire -- 40 is the aspect ratio compared to the width of 255mm -- and 17 is the diameter of tire)

The Rims and the offset numbers are where it gets foggy for me.

My Turbo Twist wheels have the following numbers on them FRONT 7 and 55
REAR 8.5 and 50. What do these numbers mean?

And the OFFSET is a complete mystery to me. How do I calculate the offset of a tire/rim???

Thanks in advance for any education here. AND please dont tell me to "do a search" or some other smart answer. I did a search and the info I found was limited and confusing.