Offset is how far the seating surface of the wheels is from the center point. A 15mm offset is the same seating position from center on all wheels. If you go down in size an inch, you lose a half inch on both edges of the wheel, so the inside is 1/2" further away from the strut, and the outside is 1/2" further inboard. Also, this generally means that the spokes are closer to the caliper, but by how much depends on how much the manufacturer changes to face of the wheel at that offset in different widths.
Here's a good resource:
http://www.willtheyfit.com/
Threaded Mode