Offset, what is it and why does it matter?
 

If I swap the wheels on a classic impreza for some bigger ones say 15's to 16's do I have to take account of offsets (whatever they are). Does the same apply for 16's to 17's.

I ask as I'm going to have 3 sets of wheels- 15's with snow tyres (205/55/15), 16's (205/50/16) and some 17's which arrive this week, tyre size unknown (thanks you scooby_two).

Can I just swap them round as and when I feel like it?