All those fancy web site names mean nothing to the internet.  When you surf to a web site you are connecting to another computer.  In order to accomplish this, your computer needs to know the numeric address of that computer on the internet.  That's where DNS servers kick in.  So what's a DNS server?

Short for Domain Name System (or Service or Server), an internet service that translates domain into IP addresses. 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.example.com might translate to 198.105.232.4.

The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.