This site has a good description: http://www.usacomp.com/Offset.htm

"Offset is the distance from the center line of the wheel to the wheel bolt up face (the part of the wheel that contacts the car's hub when you attach the wheel)"

If the mounting hole pattern and the offset are the same**, it is fairly likely you can interchange the wheels regardless of brand. There may however be some differences, if my memory serves me correctly, for instance some German cars us a bolt that has a ball shape on the head, some Japanese cars have bolts with a flat head bolt with no bevel at all. These are rare though.

**In some cases you may be able to use spacers to correct the offset but this is less desirable.

I recently bought some aftermarket wheels that were supposed to fit my Miata and they do except that the lug nuts only turn a few times before they torque up and so I'm going to have to change out the studs with some that are slightly longer.
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'92 240 + '96 960