What are NS Record's ?
NS (Name Server) Records are used within the Domain Name System (DNS) to determine which name servers are authoritative for a domain. NS records also provide name server delegation.
Within this example we have a domain named example.com. The parent zone (.com) will contain the NS records for this zone.
Now, if we wanted to create a sub domain, lets say 'sub.example.com' and host the zone on name server 'ns1.fir3net.com', we would add an additional NS record to the zone example.com defining this name server delegation.
Glue records are used to prevent circular dependencies. Circular dependencies occur if the name server referred to is under the domain that it is authoritative for.
An example is when you have a domain called example.com. Your name servers are ns1.example.com and ns2.example.com. A client tries to resolve www.example.com. The client (via the TLD servers) receives the name servers that are authoritative for this domain (which are ns1/ns2.example.com). In order to locate ns1/ns2, the parent domain (example.com) first needs to be resolved. However resolving example.com (once again) results in the authoritative name servers being returned (ns1/ns2.example.com) and in turn a circular dependency is created.
Glue records are supplied by the TLD servers within their responses i.e the name servers are supplied, but additionally the IP addresses of these name servers are also supplied. (Because this additional information is supplied by the TLD servers) Glue Records are created via your domain registrar.
Stealth Name Servers
A Stealth Name Server is a name server that is defined within a zone but it not defined on the parent name server(s).