well i really want to learn what it means when your looking at a rim and it says for example: -18mm offset and 4 and 3/4 backspace.

What exactly does this mean? thanks

 

Offset is the distance from wheel the where the wheel bolts to the hub to the center of the wheel. Backspacing is much easier to understand and visualize. It's the distance from where the wheel bolts to the hub to the rear edge of the wheel. They are just two different ways of measuring the same thing. For instance an 8 inch wide wheel with 0mm of offset would work out to be 4 inches of backspacing. Here's a pic showing how to measure backspacing.