DNS means Domain name Server or Domain name system.DNS helps to convert the ip address into domain name.Domain name into ip address.You cannot remember many website ip address at same time.So domain name is used for remembering the website.
For example 74.125.239.17 is one ip address of Google.com.It is very difficult to remember always.So you can use domain name to view the particular website.The primary work of DNS is if you give ip address in browser ,it search for domain name of particular ip address.If you give domain name in the address bar of the browser it convert the domain name to ip. If you have any queries regarding DNS visit the site http://www.whoisxy.com/

The Domain Name System (DNS) is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most importantly, it translates domain names meaningful to humans into the numerical identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide. source