The role of DNS on a computer system is to translate hostname into IP address just like a name pointing to a number entry in a phonebook. An example of hostname is www.blgr.info converted into an IP address of 188.8.131.52. DNS is also very important in email routing.
The following is a list of fast, reliable and/or free public DNS servers, that you can use in lieu of the DNS server set by your ISP. Just save your old DNS server IPs if you want to revert to old settings.
How to setup the DNS?
On *nix systems, just open /etc/resolv.conf file and add/replace above IP address (example is OpenDNS)
If you are using a Windows system, try this guide.