A set of authoritative DNS servers for a particular domain is often divided into one primary and one or more secondary ones. The primary server is the main carrier of information, so it is the place where the main data store is and where the administrator makes DNS modifications. As soon as changes are made to the settings of a domain on this server, the secondary servers synchronize these changes onto themselves with special DNS mechanisms (notification and AXFR transmission). These mechanisms are optional – it is up to the administrator to ensure that all servers have up-to-date data.
It is important to realize that dividing servers into “primary” and “secondary” is irrelevant to a person who is not the administrator of those servers. They should all contain the same information and answer any query in the same way, no matter which one you choose. Of course, this is only true if the servers are set up and synchronized correctly. It is wrong to assume that address translation takes place using the primary DNS server and the secondary will be used only if the primary is down.
For the record, the primary DNS server for the root domain is a.root-servers.net and ns.tld.cz server for the cz domain.