Wheel Tech - Offset
Wheel sizing - Wikipedia, the free encyclopedia
It's a positive 60mm. The aftermarket rims are usually "less" than that (I've seen 45mm and most are 30mm).
As the rim gets wider and wider, the offset will adjust so that you push the rim further and further out, which tends to adversely effect the handling/turning, etc.
The stock setup can fairly easily handle an inch increase (18"x9" vs. the stock 18"x8") but anything beyond that begins to put different stresses on the stock suspension, so you have to adjust the suspension/steering/turning components.