All DNS lookups for unknown hosts are first sent to one or more of the root servers. There are two layers of DNS servers at the root. The first layer is InterNIC's responsibility. The second layer is the responsibility of the domain name registrar. The InterNIC servers used to be the only servers in the process, but when the contract for the monopoly the government granted Network Solutions ended, another layer was needed to allow DNS queries to be directed to the correct registrar's servers.
The root servers are needed because a DNS lookup (also called a 'query') has to start somewhere, and the root is where things start. To perform a DNS lookup of a host for which your DNS server is not authoritative, you need the IP address of a root server in order to communicate with that root server. If you fail to connect to the root DNS server because you don't know its IP, you can't look up any DNS name you aren't authoritative for, so the root servers IP addresses are always stored locally and are shipped in a file in the DNS software (BIND/Windows DNS) and rarely change.
ROOT DNS RESOURCES
- INTERNIC: ROOT DNS CACHE FILE aka 'root hints' file (make sure you have an up to date copy of this file in your DNS server's configuration).