Some vehicles have hub centric wheels. For these, some aftermarket rims require a centering ring in order to get the wheel against the hub right.
All the wheels I have ever seen/owned were lug centric.
These just differ in the way that the wheel is centered. The lug centric, you put all the lugs on and they are tapered at the bottom. As you tighten them, the tapers push the wheel around so that it is properly spaced between each lug. The hub centrics require a ring of some type on the back of the wheel so that is lines up with the hub. My understanding is that the lugs on those wheels aren't tapered.