Q:

What is root domain?

I'm taking a class in system administration and i find it hard to understand the concept behind Root domain.
Can somebody explain in simple term the definition of Root Domain?

A:

"Domain" is a highly overloaded term: there are DNS domains, NIS domains, Windows Active Directory domains, Xen virtualization domains, and so on. 
But from the tags on your question, I assume you meant DNS domains.
When you set up a resolving DNS server, it usually comes with a built-in list of root DNS servers. When the DNS server is started, it uses that list to connect one of the root DNS servers and download an up-to-date version of the root DNS server list. This list can also be found at: ftp://ftp.internic.net/domain/named.cache
The "root domain" is the absolute highest level of the DNS hierarchy: it contains a group of geographically widely distributed, high-performance DNS servers whose main job is to know the addresses of the DNS servers of the various Top Level Domains (TLDs) like .com, .net, .edu, .us, .ca, .eu, .uk, .ru, .cn, .jp, .au, .br or .arpa. They also maintain an up-to-date list of all root domain DNS servers.
When a resolving DNS server has just been started, it knows only the addresses of the root domain nameservers. When it gets a request to resolve, say, example.com, it will first contact one of the root nameservers and requests a list of .com nameservers. It will cache this list, so that it won't have to request the same list again too often. Once it knows the addresses of .com TLD nameservers, it picks one of them, contacts it and requests from it a list of example.com nameservers, from whom it can finally request the information its client originally asked.
For a regular user, the root DNS domain is pretty much invisible. When you're learning DNS administration, you'll learn that if you want to explicitly specify that a DNS name is fully qualified, you add a full stop at the end, i.e. example.com. is explicitly fully qualified, example.com might or might not be. In this style, the fully-qualified name for the root DNS domain is just .

