The primary name server is the first server used for information about a domain as well as the location of resources associated with a domain. The information stored on the primary and secondary name servers is exactly the same. The primary server acts as the master of information control while the secondary name server periodically copies primary server information.
The way that the two interact can be explained in the following scenario: You want to find a movie in a rental store and you need to ask someone where to find it. You look to the nearest employee but they are very busy. You find a second employee who is not so busy and they are able to direct you much more quickly. The secondary name server is like a back up system to the primary name server in case the primary server is too busy or is not able to give a response.
It has become common Internet protocol to require 2 name servers for each domain.