DNS stands for Domain Name System, an Internet service that 
translates domain names into IP addressess. Because domain 
names are alphabetic, they're easier to remember.  The 
Internet however, is really based on IP addresses.  Every 
time you use a domain name, therefore a DNS service must 
translate the name into the corresponding IP address. For 
example, the domain name www.sudheer.com might translate to 
192.168.252.101.

It is very difficult to remember a set of numbers(IP
address) to connect to the Internet.The Domain Naming
Service(DNS) is used to overcome this problem.It maps one
particular IP address to a string of characters.
  FOr example, www.allinterview.com implies com is the
domain name reserved for commercial sites,allinterview is
the name of  the company and www is the name of the specific
computer,which is allinterview's server.