A DNS server controls one or more zones, meaning that it serves records for names in those zones, for the most part, and the parent zones' servers have delegated authority for those zones to the child server. E.g., a name server for the edu domain, seeing a query for the name somewhere.cwru.edu , would respond with an NS record giving the name server for the cwru.edu domain. The record type, class, and query ID are encoded as 16-bit numeric values. A failed request might occur if the domain name does not have any records of the requested type, or if the name does not exist. Responses distinguish between these conditions.
Zone transfers are a way of copying data from a primary server to secondary servers.
A secondary server polls the primary, requesting the SOA record for the root node of the zone. It checks the serial number in this record. If the serial number has increased since the last zone transfer, the secondary sends a query for the root node, with AXFR as the query type. The primary server responds with a list of all records in the zone. There have been improvements to this strategy, involving notifying secondaries when records have changed so that polling is unnecessary, and using incremental transfers instead of sending the whole zone each time a change is made, but there are usually much easier ways to mirror this data.