Well i was searching to find out what the EXACT definition of a wheel's offset is in the forums but really couldn't put a picture to a definition... yeah it's easier to picture sumfing to understand it sumtimes. So i went googling and this is wot i found taken from http://www.conceptonewheels.com/fitment.html

Here's the snippet i found most useful...

Backspacing is simply the distance form a wheel's hub mounting surface to an imaginary line across its diameter. Offset, usually measured in millimeters, is the distance from the hub mounting surface to the cetner line of the wheel. In other words, a wheel's hub mounting surface can be located either inside or outside of its center line. If the surface is outside (toward's the wheel's face) the centerline, then the offset is positive. Most modern front wheel drive cars requires a positive offset. SUVs usually feature a negative or zero offset. Negative offset places the tires outside of the fender well and are are usually used in off road applications where wide off road tires are needed and track width increased.