[ Thanks to Andrew
Weber for this link. ]
“The primary purpose of the slave server is to provide a backup
to the primary master server should it go down for any reason. The
major difference between the master server and the slave is where
they get their data. The primary master server gets it’s
information from zone files that are created on it. The slave
server gets data from the master server which provide it with the
necessary information. When a slave server receives this
information from a server it is called a zone transfer. Typically,
in order to differentiate the master zone files from the slave, the
names on the slave are changed so they start with “bak” to indicate
they are really backup files.”