Primary and Secondary

Primary name server
has the "master" zone file
operator modify the zone file in the primary name server
typically only one primary server exists for a zone

Secondary name server
has a copy of data stored in the primary server
can be regarded as a "backup" of primary server
redundancy / load-balancing

No difference from client (resolver)'s point of view
both primary and secondary servers return the same response
(module zone transfer timing)