DNS zone transfer, also sometimes known by its (most common) opcode mnemonic AXFR, is a type of DNS transaction. It is one of the many mechanisms available for administrators to employ for replicating the databases containing the DNS data across a set of DNS servers. Zone transfer comes in two flavors, full (opcode AXFR) and incremental (IXFR). Nearly universal at one time, it is now falling by the wayside somewhat, in favor of the use of other database replication mechanisms that modern DNS server packages provide.
source:
Wikipedia: DNS zone transfer
more info:
Microsoft TechCenter: Understanding zones and zone transfer
RFC1034: Zone Transfer in DNS
RFC1995: Incremental Zone Transfer in DNS
How the AXFR protocol works
nslookup and DNS Zone Transfers
