Name servers that hold all information on hosts within a zone are called authoritative for this zone, and are sometimes referred to as master name servers. Any query for a host within this zone will finally wind down at one of these master name servers.
To provide a coherent picture of a zone, its master servers must be fairly well synchronized. This is achieved by making one of them the primary server, which loads its zone information from data files, and making the others secondary servers who transfer the zone data from the primary server at regular intervals.
One reason to have several name servers is to distribute work load, another is redundance. When one name server machine fails in a benign way, like crashing or losing its network connection, all queries will fall back to the other servers. Of course, this scheme doesn't protect you from server malfunctions that produce wrong replies to all DNS requests, e.g. from software bugs in the server program itself.
Of course, you can also think of running a name server that is not authoritative for any domain. This type of server is useful nevertheless, as it is still able to conduct DNS queries for the applications running on the local network, and cache the information. It is therefore called a caching-only server.