NS Record

NS-records identify DNS servers responsible for your zone.

A zone should contain one NS-record for each of its own DNS servers (primary and secondaries).
This mostly is used for zone transfer purposes (notify). These NS-records have the same name as the zone in which they are located.

The most important function of the NS-record is delegation. Delegation means that part of a domain is delegated to other DNS servers. For example all ".com" sub-names (such as "microtech.com") are delegated from the "com" zone (hosted by the "InterNic"). The "com" zone contains NS-records for all ".com" sub-names (a lot!).

You can also delegate sub-names of your own domain name (such as "subname.yourname.com") to other DNS servers. You are in effect the "InterNic" for all sub-names of your own domain name

To delegate "subname.yourname.com", create NS-records for "subname.yourname.com" in the "yourname.com" zone. These NS-records must point to the DNS server responsible for "subname.yourname.com" for example "ns1.subname.yourname.com" - or a DNS server somewhere else like "ns1.othername.net".

An NS-record identifies the name of a DNS server - not the IP-address.
Because of this, it is important that an A-record for the referenced DNS server exists (not necessarily on your server, but wherever it belongs), otherwise there may not be any way to find that DNS server and communicate with it.