This subject had confused me for a great amount of time myself. While I am still not entirely sure of this, the way I understand it is this. The backspacing on a wheel is the distance from the farthest outside point on the lip of the rim, to the surface which the bolts rest to tighten the rim down. Thus you would have your backspacing. Offset I believe is measured from the center of the rim as opposed to the edge. The offset of the rim is the measured distance from an imaginary line running down the center of the rim longitudinally to the surface which the bolts tighten down to attach the rim. I'm not entirely positive on these definitions or if perhaps I have them swapped up or what not. It's just what I can recollect from the cob webs of my spam filled college students brain.
it sounds a lot more complicated than it really is. when you see a car with wire wheel sticking out of the fenders, its commonly referred to as having negative offset wheels, meaning the mounting surface of the wheel is way to far in on the barrel of the wheel. hope this helps a bit.