Each primary and secondary zone of the same name on two separate DNS servers
contains a serial number. Peroidically a check is made to determine if the
serial number of the primary matches the secondary. If they do, nothing
happens. If they don't then the secondary zone is locked and the secondary
zone gets the changes (i.e. adds, deletes and changes) to the zone. Note
that I said changes as this makes the update process quicker. This process
repeats over and over again day after day.
A reload from master is a manual process kicked off by executing a
DNSCMD.EXE command line request or by requesting the process though the DNS
Managment Console on the server performing your DNS functions. When such a
request is made the secondary copy get a full and complete reload of the
zone from the primary without regard to the serial numbers or the time.
Reloads can be done whenever and however frequently you want.
A reload of the Primary zone is typically performed from the Managment
Console. When you invoke the console and display a zone the records in the
zone are cached. When you request a reload you are asking that the cache be
completely replaced.