The NS, or Name Server records of a domain, point out which servers deal with the Domain Name System (DNS) records for it. Setting the name servers of a particular hosting provider for your domain address is the most convenient way to point it to their system and all its sub-records are going to be handled on their end. This includes A (the IP address of the server/website), MX (mail server), TXT (free text), SRV (services), CNAME (forwarding), etcetera, so if you want to modify any of these records, you will be able to do it using their system. To put it differently, the NS records of a domain address reveal the DNS servers which are authoritative for it, so when you try to open a web address, the DNS servers are contacted to obtain the DNS records of the domain you want to access. This way the website that you're going to see will be retrieved from the correct location. The name servers normally have a prefix “ns” or “dns” and each domain address has at least 2 NS records. There is absolutely no sensible difference between the two prefixes, so which one a web hosting provider is going to use depends solely on their preference.