Domain
Naming
System
All websites are really IP Addresses on the internet.
goto your command prompt and type Ping (anywebsite) thats their true URL.
So you can goto www.google.com 2 ways
in your Address bar you can type.
1. 74.125.67.106
or
2. www.google.com
Same website. DNS converts IP Addresses of websites into Easy to Remember Host names
You can read more here
http://en.wikipedia.org/wiki/Domain_Name_System