The Name Servers of a domain name reveal the DNS servers that deal with its DNS records. The IP address of the site (A record), the mail server that handles the e-mails for a domain (MX records), any text record in free form (TXT record), directing (CNAME record) and so on are obtained from the DNS servers of the website hosting provider and for any domain to be using them and to be forwarded to their hosting platform, it has to have their name servers, or NS records. If you would like to open a site, for example, and you type the URL, the web browser connects to a DNS server, which keeps the NS records for the domain and the request is then sent to the DNS servers of the hosting provider where the A record of the site is retrieved, so you can look at the content from the correct location. Commonly a domain name has a couple of name servers that start with NS or DNS as a prefix and the contrast between the two is simply visual.