The Domain name server also known as  "DNS" are the servers or say machines that finds a particular "IP-ADDRESS" from the domain name. In other words, Domain Name Servers  (DNS) are the Internet's equivalent of a phone book. They maintain a directory of  domain names   and translate them to Internet Protocol (IP) addresses. This is necessary because, although domain names are easy for people to remember, computers or machines, access websites based on IP addresses.

For better understanding I am giving you a Example below that will show you what is a domain name and how it is connect to the IP-ADDRESS.

EXAMPLE :- www.google.com

Above here is a link that would throw you to the Google Web Page. Here "google.com" is the "domain name" and ".com" is the top-level domain name.  So this domain name would have a unique "IP-ADDRESS" on the internet and the DNS will search the internet for a IP-ADDRESS have the specified Domain Name.

Keep in mind that every single "domain name" have a unique "IP-ADDRESS" or say Internet Protocol (IP) Address.

Further Information on DNS Visit: Wikipedia/DNS