The Name Servers of a domain reveal the DNS servers that deal with its DNS records. The Internet protocol address of the site (A record), the mail server that deals with the e-mails for a domain name (MX records), any text record in free form (TXT record), directing (CNAME record) etc are extracted from the DNS servers of the website hosting provider and for any domain name to be using them and to be directed to their hosting platform, it should have their name servers, or NS records. If you want to open a site, for instance, and you enter the URL, the browser connects to a DNS server, which keeps the NS records for the domain name and the request is then forwarded to the DNS servers of the hosting company where the A record of the web site is retrieved, allowing you to view the content from the correct location. Usually a domain has a couple of name servers that start with NS or DNS as a prefix and the distinction between the two is simply visual.