The Wheel and Tire Bible
Tons of good info... if you've got 10 or 15 minutes, read this over...
or just skim. I'm confident you'll learn at least one new thing 
http://www.carbibles.com/tyre_bible.html
Wheel Offset and Back Spacing
Backspacing
This is the distance from the back edge of the wheel to the hub mounting surface.
To determine the wheel backspace:
Position the wheel face down.
Lay a straight-edge across the back of the wheel.
Measure the distance from the straight-edge to the wheel’s hub mounting surface.
Offset
A wheel’s offset is the distance from its hub mounting surface to the centerline of the wheel. A zero offset would place the hub mounting surface exactly on the centerline (the halfway point in the width of the wheel). If the hub mounting surface is positioned closer to the outside of the wheel it is considered more positive, and if it is positioned close to the inside of the wheel it is considered more negative.
Offset is measured in millimeters and would be displayed as ET35 or +35 for a positive 35 offset, for example.
A more Positive offset:
The more positive the offset, the more the hub mounting surface moves toward the outsid of the wheel. This generally results in a more convex spoke shape and/or less lip, and positions the wheel closer to the strut and farther from the fender.
Almost all wheels have some degree of positive offset, usually between +15 and +55 depending on the make and model of the car. Although +15, for example, is a positive offset, it is considered to be much more negative than +55 would be and would result in a more aggressive wheel fitment (wheel would be closer to the fender).
A more Negative offset:
The more negative the offset, the more the hub mounting surface moves toward the inside of the wheel. This generally results in a move concave spoke shape and/or more lip, and positions the wheel farther from the strut and close to the fender.
It is uncommon to see wheels with a negative offset value unless you are looking at widebody cars or unconventionally aggressive wheel fitment.
To determine wheel offset:
Every wheel has an offset, and every wheels offset can be measured. There are no 'secret' offsets, although some Supra owners seem to think that way.
-Position the wheel on a flat surface and measure its overall width.
-Divide the overall width by two, then subtract this result from the backspace value.
-Offset = Backspace - (Rim Width ÷ 2)
For example if you have a 10" wide wheel with 6" of backspacing, the math would be: 6 - (10 ÷ 2) = 1 inch. 1 inch = ~25mm so your offset would be +25.
(FYI: wheel width is measured from INSIDE lip to INSIDE lip, and overall width (outside to outside) is usually about 1/2" wider than measured width)
Another photo displaying different offsets:
