Go to www.usacomp.com/Offset.htm, it will teach you about offsets. MAKE SURE you know what you are doing before you buy rims. Don't get burned like I did!!! The best thing to do is to take off your OEM rims now, take a straight edge across the back of your rim. Measure the distance from the mounting serface inside the rim to the straight edge. This is your backspace. Make sure your backspace for your new rims are close to the same, otherwise something will be rubbing, etc. A way to fix that problem will be to use slip on wheel spacers (cheap and dangerous-my opinion) or bolt on wheel adapters (expensive- and I had no choice after I purchased rims without knowing what I was doing!).
Also, make sure your new rims/tires are approximate to the diameter of your OEM ones, or your ABS, Speedo, etc. will be off.
Try www.1010tires.com/tiresizecalculator.asp?...to look at how much you can screw up tires!!!
Good luck!!