Back spacing is the distance from the mounting surface of the rim to the back edge of the rim. The smaller the backspace the further the rim sticks out from under the fender. Offset I do believe is the amount from the center of the rim in one direction or the other, depending on whether the rim is sticking out or tucking in under the fender.
98 TJ 4i in currie with Currie bars front and rear
2014 JKUR AMP"D,Ace slider,BFG AT/KO's 33's
Ace Tire carrier/ rear bumper,LOD Mid width front bumper
I love my country. That's why I despise the government.