The Name Servers of a domain name reveal the DNS servers that handle its DNS records. The IP of the site (A record), the mail server that handles the e-mails for a domain name (MX records), any text record in free form (TXT record), pointing (CNAME record) and so forth are taken from the DNS servers of the website hosting company and for any domain to be using them and to be pointed to their hosting platform, it should have their name servers, or NS records. If you would like to open a website, 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 sent to the DNS servers of the hosting company where the A record of the website is obtained, so that you can look at the content from the proper location. Usually 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.