The Name Servers of a domain point out the DNS servers that manage its DNS records. The IP of the website (A record), the mail server that manages the emails for a domain name (MX records), any text record in free form (TXT record), directing (CNAME record) etc are extracted from the DNS servers of the website hosting provider and for any domain name 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 website, for example, and you insert the URL, the browser connects to a DNS server, which keeps the NS records for the domain and the request is then pointed to the DNS servers of the webhosting provider where the A record of the web 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 contrast between the two is just visual.