How they work
There are many name servers in the DNS infrastructure. Each name server contains information about a portion of the domain name space. Name servers are associated with levels as far as the first three levels of domain name space are concerned.
There are 13 name servers associated with the root level; they are called root servers. Two of the root servers are currently run by VeriSign; the rest are operated by other organizations around the world as a service to the Internet community.
The organizations that run name servers associated with a TLD are called domain name registries. Generally, ccTLDs are run by designated domain name registries in the respective countries, and gTLDs are run by global registries. For example, VeriSign currently manages the name servers for the .com and .net TLDs, a nonprofit entity called Public Internet Registry (PIR) manages the name servers for the .org TLD, and another nonprofit organization called EDUCAUSE manages the name servers for the .edu TLD. All of these registry organizations are subject to change, however.
The name servers associated with enterprise-level domains and below are either run directly by the organizations that own those domains or outsourced to Internet service providers (ISP) or other service providers.