A primary master server is a server that loads its data from a file on disk. This server can also delegate authority to other servers in its domain. A secondary master server is a server that is delegated authority and receives its data for a domain from a primary master server. At boot time, the secondary server requests all the data for the given domain from the primary master server. This server then periodically checks with the primary server to see if it needs to update its data.
Root servers are the master servers for the root and top-level Internet domains. They are listed in the root.cache file described in "BIND's root.cache File".