The NS (Name Server) records of a domain name reveal which DNS servers are authoritative for its zone. Essentially, the zone is the collection of all records for the domain name, so when you open a URL inside an Internet browser, your PC asks the DNS servers worldwide where the domain address is hosted and from which servers the DNS records for the domain should be retrieved. This way a web browser finds out what the A or AAAA record of the domain address is so that the latter is mapped to an Internet protocol address and the website content is requested from the right location, a mail relay server finds out which server takes care of the emails for the domain address (MX record) so a message can be forwarded to the correct mailbox, and so forth. Any modification of these sub-records is performed with the help of the company whose name servers are employed, permitting you to keep the website hosting and change only your email provider for instance. Each domain has no less than two NS records - primary and secondary, which start with a prefix such as NS or DNS.