Let's say you have an 8" wide rim. If you have exactly 4" to each side of the verticle flange where the wheel meets your brake rotor or drum, then the wheel has NO offset. Anything other than the mounting flange being EXACTLY centered in the wheel, you have an offset. The offset is measured in mm's.
The 5x108 mm I mentioned before referred to the bolt pattern. The taurus has 5 lug nuts, in a 108mm diameter circle. Just because a wheel has 5 lug nuts, the circle the lug nuts make is not necessarily the same diameter. The wheels you listed, I think had a 4.5" diameter.
Hope this helps.
SHO 2NV, thanks for the info on the hub centerics.
Offset is basically the clearance inside the wheel. The measurement is made so that once mounted, the wheels clears the brakes and suspension, lines up with the fenders of the car, and supports the weight of the hub evenly.