1st - fits in between the wheel & the hub and longer lug bolts go through them. Think of it as putting a washer between the wheel & the hub. Commonly used when you need to move the wheel/tire set-up out 5-10 mm, though it can be used up to 20mm or more.
2nd - bolts to the hub usually using shallow head bolts, giving you a new surface to mount the wheel/tire. Some have studs, some your exiting bolts thread into. These typically are 20mm or more, as there needs to be enough thickness to support the load of either/bolt sets of bolts.
Hope that helps.
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