The Name Servers of a domain reveal the DNS servers that deal with its DNS records. The IP of the web site (A record), the mail server that manages the e-mails for a domain (MX records), any text record in free form (TXT record), pointing (CNAME record) etc are taken from the DNS servers of the website hosting provider and for any Internet domain to be using them and to be forwarded to their hosting platform, it ought to have their name servers, or NS records. If you would like to open a site, for example, and you enter the URL, the Internet browser connects to a DNS server, which keeps the NS records for the domain name and the request is then sent to the DNS servers of the webhosting provider where the A record of the site is retrieved, allowing you to look at the content from the right location. Commonly a domain has a couple of name servers that start with NS or DNS as a prefix and the difference between the two is just visual.