An NS (name server) record allows you to delegate a subdomain of your domain to another name server.
For example you have the domain mydomain.com which is using the Domainmonster.com name servers. You decide that you want your own name server (myns.mydomain.com), for which you have already added an A-record, to look after the DNS for a subdomain on your domain: extranet.mydomain.com.
To do this, you would add an NS record for hostname/Zone "extranet" with the name server address "myns.mydomain.com". If someone therefore wants any DNS information about extranet.mydomain.com, our Domainmonster.com name servers will send them in the direction of the server at myns.mydomain.com.
