A Domain Name Server (DNS) server stores a database of different domain names, network names, Internet hosts, DNS records and other related data. The most basic function of a DNS server is to translate a domain name into its respective Internet Protocol address (IP address). During a domain name resolution query, DNS records are searched, and if found, the domain name record is returned. If the domain name is not registered or added to that DNS server, the query is then passed to other DNS servers until the domain name record is found.
Primarily, DNS is designed to locate and deliver websites to end users over the Internet or a private network, a DNS server is developed on typical hardware but runs specialized DNS software. It is always connected to the Internet or a network.
Source: Domain Name Server (DNS)