The file that contains information about a 'Zone' (a DNS domain).
For instance there is a ".co.nz" zonefile that holds information about where to look for all the *.co.nz domains.
which specifies the domain to add to a record that doesn't have one.
A Zone file can also contain comments, which are lines that start with a semicolon.
Otherwise a zone file contains 'resource records' which are an optional name, an optional TTL, an optional class (The only one you'll ever use is IN for Internet, others exist such as CH for Chaos, but noone uses them), the resource record type and any parameters it requires.
If the name is omitted then the name of the previous record is used, if the TTL is omitted the default TTL (as specified by $TTL) is used, the default class can be specified in the name server configuration.
All zones must start with an SOA ResourceRecord.
You can use a "@" to refer to the current zone instead of explicitly typing out a hostname.