Domain Name Server maps the web addresses you write in the address bar of the browser to the actual IP address of the server.
Many people are not even aware of these settings because when you configure your DSL / Broadband connection. The service provider updates the settings , either in the router or in the Network Settings.
You can always overwrite the modem / router settings by entering your own DNS servers in the LAN settings. Google open DNS is fast, safe , and free!
To access your LAN settings, Go to Control Panel -> Network Connection
Right click on Local Area Connection and go to properties
Select TCP/IP and go to properties
Update the DNS settings as shown (Do not change the settings other than the marked ones in the picture)