Standard Primary "Master Database".
The DNS data is kept in a database that can be stored in a text file or in the active directory database when the DNS service is configured on a domain controller. The DNS data is organized into zones; each zone is a specific portion of DNS namespace that is stored in a separate file or as a unit of replication when stored in active directory.
DNS servers can host one or more zones of a particular domain. When creating an active directory domain, a corresponding DNS zone with the same name as the new domain must exist or be created during the process to ensure proper functionality of the directory services. DNS zones contain different resource records. Resource records specify a resource type, and the IP address to locate the resource. DNS zones can resolve names to IP addresses or IP addresses to names for devices running the TCP/IP protocol like workstations, servers, routers, switches, etc.
Secondary Read-only copy of a zone containing specific resource records
A secondary zone is a read only copy of the DNS data file, but still provides the ability to answer DNS name resolution queries from clients and other servers authoritatively for the portion of the DNS namespace it is setup for. These zones help ease the excessive traffic a primary zone may otherwise receive, and can answer queries if the primary server fails.
The primary zone acts as a master zone for a secondary zone, and a secondary zone can act as a master zone for another secondary zone. Secondary zones update their data by querying master zones at pre-set time intervals and if data has changed a zone transfer to the secondary updates its data.
Help reduce excessive network traffic.
When more than one DNS Server is deployed, data consistency between the DNS servers and their zone information is important. To provide synchronization and current information for all DNS servers you need to ensure DNS zone transfers\replication occurs.