A Zone is a contiguous portion of DNS namespace managed by one or more name servers. Zones contain resource records that specify the name of the DNS server authoritative for the zone (SOA record), the names and IP addresses of all name servers in the zone (NS records), the names and IP addresses of other hosts (A records), aliases for hosts (CNAME records), and so on.
Holds Read and Write copies of all resource records (A, NS, _SRV).
Read only copies of records, gets updates from the primary server by zone transfer.
New in Windows 2003, a sub zone with just pointers to another domain. NS, SOA and A record of the main server will be in Stub dns.
If the clients in the domain often query the ip address and computers of the other domain (abc.com), then we can configure that DNS server which resolves the dns query as a conditional forwarder in this domain