The Nazgûl used be just regular human kings, but the nine rings given to them by Sauron turned them from normal-powerful to super-magical-powerful. While they gained wealth they lost their humanity, eventually becoming specters visible only to those in the realm of wraiths (which is why they weren't fooled when Frodo puts on the Ring at Weathertop and why every time he puts on the Ring, they sense his presence).
The Nazgûl are a pretty stunning lesson in the dangers of greed and accepting gifts from total strangers. But what's more interesting is that there are nine of them. This is also the number of people in the fellowship: four hobbits, a dwarf, an elf, two men, and a wizard. A coincidence? Hardly. These wraiths are the evil twin of the fellowship, representing the consequences of heeding the Ring's siren song.