Need new rims, what size?

I need new rims on my -98 explorer, here in sweden.
I don't speak the "rim language"...

What do i need to know regarding dimensions etc?
Info found so far (in the net): 5/114,3 & ET 12.0.

When looking for new rims other values are presented.
Such as 7,0x15 and 8,0x17...
Is there a conversion table or something?

I used the original rims for winter tires with spikes,
and now i need new ones for the original M/S tires still good for use (255/70R16).

Martin/