From beginlinux.com
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.
