[usurp]

Hey guys, my OEM z4 wheel cracked and another is dented so i decided to order some new wheels from tirerack. Now I put in all the details 2005, BMW, Z4 3.0 and its displaying a bunch of wheels for me. I am stick to 18" but every wheel i click seems to have a different offset and backspacing. What do they mean?

I found for example some BBS RX with the following specs:
Offset: 38mm Backspacing: 6.0" Bolt Pattern: 5-120

I then checked the BBS CK and it had the following specs:
Offset: 45mm Backspacing: 6.29" Bolt Pattern: 5-120

What does it mean?

thanks!