what's up with wheel offset?

I'm looking for aftermarket wheels for my Suburban.  I'm finding that there are different offsets of the wheels from zero to +26.  I figure this may affect the looks of the wheels - but does it affect the performance of the tire and the suspension?  It seems that changing the offset on the front wheels will mess up the steering geometry.  Does anyone have some answers on this?  Thanks for your help.

Here's some information:
 
http://www.miata.net/garage/offset.htm