Authoritative vs. Caching Nameservers

Authoritative Nameservers are the ultimate authority for a domain.  All other DNS servers on the Internet should go to it for information about the domain.  Authoritative nameservers usually only have one job, to be authoritative.  When you register a domain name, you set the DNS for your domain to point to authoritative nameserver.  Not only does it hand out resource records for a domain, but also tells caching servers how long to cache a particular response.

Caching DNS Servers query authoritative nameservers (or other caching DNS servers) and return the results based on the answer of an authoritative nameserver.  Caching servers are what all end-users point their DNS to when they configure their machines.  This is because authoritative nameservers will not give you the address of Google or Yahoo; they only know about the domains for which they are authoritative.