Diff. between backspace and offset?

I'm planning a 5-lug swap this winter along with traction-loc rebuild and 3.73 gears. Im going to use the Ford Racing Cobra R COMPLETE 5-lug swap for $1800 because it comes with all the parts and intructions and I don't have to track down parts (hard to find Mustangs in junkyards around here). Well I'm going to order the new wheels and tires soon...the wheels I looked at were listed like this:

5-lug (5X4½). They are 17" x 9", +26mm offset. The backspace is 6.11

What is the difference between offset and backspace? I don't want to order the wrong wheels and have to leave the car on the lift! Will the width of my rearend change with the Cobra R conversion? Thanks for any help.

Me too...but I've been looking and several companies, incl. American Racing, have them both listed apart.?.?

Thank you very much...that is exactly what I need...I can tell how much of a dish the wheel will have now. Thanks.