Domain Name System (DNS) is a system of mapping names to IP addresses for various purposes. This system comprises a multilevel server systems, and client (resolver) software built into most IP enabled operating systems.
The domain name structure is a tree of domain names with multiple levels of authority. The DNS top-level "Root" servers are a set of DNS servers that are authoritative for the serving the ".com" or ".org" domain.
There is an "assumed ." at the end of each domain name, so www.cisco.com is really www.cisco.com. This is know as a Fully Qualified Domain Name (FQDN)
A company can use a name space registrar (i.e. godaddy, verisign, etc) to "reserve" a domain name under one or more of the root domains (i.e. cisco under .com = cisco.com)
When a computer on the Internet wants to resolve a domain name, it works from right to left, asking each name server in turn about the element to its left. The root name servers (which have responsibility for the . domain) know which servers are responsible for the top-level domains. Each top-level domain (such as .com) has its own set of servers, which in turn delegate to the name servers responsible for individual domain names (such as example.com), which in turn answer queries for IP addresses of sub-domains or hosts (such as www).
Entries in DNS are normally entered manually in the DNS service who controls the "primary zone". Each dns zone can have multiple "secondary" copies on other DNS servers for high availability, efficiency, and redundancy.
Some example of DNS entries are
SOA - (Start of authority) header information on a DNS zone
A record - mapping of a name to the IP address
PTR record - mapping of IP address to name
CNAME record - mapping of a name to another name (like an alias)
MX record - mapping a domains "mail exchange" to a email servers
TXT record - mapping some text to a IP address
other types of DNS zone entries also exist.
The DNS service runs on TCP/UDP port 53. (UDP is preferred for client to service communication)