The Domain Name System uses a hierarchical system to create a name database to provide name resolution. The hierarchy looks like an inverted tree with the root at the top and branches below.
At the top of the hierarchy, the root servers maintain records about how to reach the top-level domain servers, which in turn have records that point to the secondary level domain servers and so on.
The different top-level domains represent the either the type of organization or the country or origin. Examples of top-level domains are:
.au - Australia
.co - Colombia
.com - a business or industry
.jp - Japan
.org - a non-profit organization
After top-level domains are second-level domain names, and below them are other lower level domains.
Each domain name is a path down this inverted tree starting from the root.
For example, as shown in the figure, the root DNS server may not know exactly where the e-mail server mail.cisco.com is located, but it maintains a record for the "com" domain within the top-level domain. Likewise, the servers within the "com" domain may not have a record for mail.cisco.com, but they do have a record for the "cisco.com" domain. The servers within the cisco.com domain have a record (a MX record to be precise) for mail.cisco.com.
The Domain Name System relies on this hierarchy of decentralized servers to store and maintain these resource records. The resource records list domain names that the server can resolve and alternative servers that can also process requests. If a given server has resource records that correspond to its level in the domain hierarchy, it is said to be authoritative for those records.
For example, a name server in the cisco.netacad.net domain would not be authoritative for the mail.cisco.com record because that record is held at a higher domain level server, specifically the name server in the cisco.com domain.
Links
http://www.ietf.org//rfc/rfc1034.txt
http://www.ietf.org/rfc/rfc1035.txt