The Name Servers of a domain reveal the DNS servers that manage its DNS records. The IP address of the web site (A record), the mail server that handles the emails for a domain address (MX records), any text record in free form (TXT record), directing (CNAME record) and so on are taken from the DNS servers of the web hosting provider and for any domain address to be using them and to be directed to their hosting platform, it ought to have their name servers, or NS records. If you wish to open a website, for instance, and you input the URL, the Internet 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 webhosting provider where the A record of the site is retrieved, allowing you to view the content from the proper 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 simply visual.