greetings!

I have an 84 vanagon GL with 14" wheels and am looking at purchasing either a spare set of 14's or a set of 15's.  I currently have two sets of tires (winter/summer) and am thinkng of making one a 15" set (eventually both 15" sets in the future perhaps...)

I'm looking at purchasing a used set of 15" steel rims 15x6, 5 lug 112mm...
can someone please explain to me the difference and compatability between the 15x6 and the 14x5.5?  thanks!