DNS (Domain Name System) Servers can be generally classified into two types; 1) Primary DNS Server 2) Secondary DNS Server.
Primary DNS Server: A Primary DNS Server is a server holds the Primary Zone file which it has authority. Primary Zone file is a Read-Write copy of DNS Zone file. Resource Records for the Zone are created, updated and maintained, in Primary Zone file.
Secondary DNS Server: A Secondary DNS Server is a server that holds the Secondary Zone file, which is a Read-Only copy of the Primary Zone file. It is not possible to create or update Resource Records in a Secondary Zone.
The Secondary DNS Server transfers the information about a zone from another DNS Server (Primary or Secondary) via a process called Zone Transfer. Any update is required in the Zone, must be done at the Primary DNS Server and then transferred to the Secondary DNS Server.