Name Server is a computer server that hosts a network service for providing responses to queries against a directory service. It maps a human-recognizable identifier to a system-internal, often numeric, identification or addressing component. This service is performed by the server according to a network service protocol.
eg of name server is DNS (Domain Name System) .The important function of the DNS is to change the domain name in to Ip address.
You can get the Reverse DNS lookup PTR (Pointer Record Lookup) at here
whoisxy.com and know it' detail.