Due to the fact that DNS is so important in the workings of the Internet, it would make no sense to have only one name server per domain or zone.
As a result, DNS servers generally work in pairs - a master name server and a slave name server. Master name servers are, in fact, no more or less important than slave servers in their job or answering queries. Their primary difference is that the master server is the one that contains all the zone files. we'll get to set up a master shortly, so hang in there.
Since it would make no sense to maintain two copies of the zones files (one on the master and one on the slave), on startup, the slave server will do a zone transfer from the master. In effect, this obviates the need for maintaining two sets of files on two machines.
When the master server starts, it reads the zone files, and immediately begins answering queries. When the slave starts up, it transfers the zone information from the master, and only once it is complete, does it begin answering queries.
In the event that the master is not available when the slave starts up, BIND can be configured to read the zone file from a backup that was stored on the last zone transfer.
Of course, there has to be a timeout on the slaves information otherwise the slave could continue answering queries about a host long after the host has been reconfigured or decommissioned.