Post informative offset links or quotes...
#/forums/cgi-bin...m=2&topic=1471

http://www.tirerack.com/wheels/tech/offset.htm

(edit*)Courtesy Matt (mdsdm) from 240sx.org
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original wheel width = a (in inches)
original wheel offset = b (in mm)
new wheel width = c (in inches)
new wheel offset = d (in mm)
difference in clearance on the inside between original & new = f (in mm)
difference in amount the wheel sticks out bewtween original & new = g (in mm)

f = ((a*25.4)/2 + b)-((c*25.4)/2 + d)
g = ((a*25.4)/2 - b)-((c*25.4)/2 - d)

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Here's more info:

-Offset is the distance from the front of the hub-mounting surface to the center of the wheel.
-Zero offset means that the hub mounting surface is even with the centerline of the wheel.
-Positive offset means that the hub mounting surface is toward the street-side of the wheel.
-Negative means the opposite. (towards the axles).

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Edit: Never mind, I just read Lance's link.