The Name Servers of a domain reveal the DNS servers that handle its DNS records. The IP of the website (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 taken from the DNS servers of the hosting provider and for any Internet domain to be using them and to be pointed to their hosting platform, it should have their name servers, or NS records. If you wish to open a site, for example, and you type the URL, the browser connects to a DNS server, which keeps the NS records for the domain name and the request is then pointed to the DNS servers of the hosting company where the A record of the website is obtained, enabling you to see the content from the right location. Normally a domain address has a couple of name servers that start with NS or DNS as a prefix and the difference between the two is simply visual.