It's just a prepaid credit card. Looks and behaves like any normal credit card, except that it has no name or address associated with it (and will validate against any).

Yes, they have unique numbers, and the time/date/location of purchase is known for each card's number. Like I said, this is not secure enough to defend against targeted attacks by well-resourced actors, but good enough to stay out of the dragnet, at least for now.