What is the Domain Name System?
When you use your web browser to connect to a website, your computer uses the DNS system to find the numerical address of the server on which the website is contained. Within the website address is a portion that is called the 'domain name'. The system designed to look this domain up and find the correct server address is called the Domain Name System. The DNS is a system what allows computers to turn a domain names into the unique numerical addresses of the sites and systems you wish to connect to. When you type "http://www.yahoo.com" into your web browser, your browser sends a DNS query to request that the DNS system look up the exact IP address of Yahoo's web server.
The Domain Name System consists of a centralized database that maps domain names to servers that can provide resolution for that domain. The central servers are often referred to as the root Network Information Center (root NIC) or simply the Internet NIC (InterNIC). This database must be centralized because there are millions of users out there and they all must have a single source for information, a central place to start when looking for one of those unique IP addresses.
To get your domain name into this central database at InterNIC, you must register your Domain Name, and provide the required information, including your DNS servers. This is done by contacting a Domain Name Registrar, giving them your billing, address and DNS server information. For a fee these registrars will insert your domain into the central database at InterNIC.
The DNS is also a hierarchical system. Domain names are organized into levels. The InterNIC handles many of the top level domains. The second and third level domains are handled at DNS servers that are usually located at the Domain owner's site. This means that queries only go to the root once, and only if the local DNS server doesn't already manage that domain.