http://www.ntchosting.com/dns/primary-and-secondary.html
Primary DNS / Secondary DNS
DNS, or Domain Name Server, represents a computer, which is charged with the task of translating domain names into IP addresses. Thanks to the DNS service we can type access websites by only typing their alpha-numeric names (domain names) in the browser instead of their IP addresses, which are the real server-side names of the websites.
There are two basic types of DNS - primary DNS and secondary DNS, which are generally applied to each newly registered domain. They are name server computers where the record of your domain name is stored. The information on both servers is identical. The two DNS values are usually configured by your ISP or hosting provider and given to you as a very important part of your domain registration order details.
In general, domain names can work with only one name server - the primary DNS. However, practice has shown that a domain name needs to have at least two name servers assigned in order to be available at any time. In case there is a problem with the primary name server, the secondary name server will be able to answer the online request for a particular domain. This back-up requirement has turned into an accepted Internet standard that prevents domain names from going offline.