DNS - understanding zones
Domain Name System (DNS) data is divided into manageable sets of data called zones. And each of these sets is a specific zone type.
Zones contain name and IPv4 or IPv6 address information about one or more parts of a DNS domain. A server that contains all of the information for a zone is the authoritative server for the domain, called a parent zone. Sometimes it makes sense to delegate the authority for answering DNS queries for a particular subdomain to another DNS server, called a child zone. In this case, the DNS server for the domain can be configured to refer the subdomain queries to the appropriate server.
For more information on groups of zones, take a look at DNS - What is a delegation zone.