Domain Names:
A Domain Name is a UNIQUE identifier to find a resource (website-email-ftp etc.) on the internet, no two domain names are the same. High level domains are .com .net .org etc.
More Detail:
Think of the Internet in the way the post office works. EVERY SINGLE MACHINE that connects to the internet has an address, called an IP address. Even the machine you are on right now has your very own address. An IP address is formed from four octets (8 binary digits) like this 192.168.123.132 or as the machines see it in binary 11000000.10101000.01111011.10000100. Computers use this to find other computers, so it builds from left to right. Think of it as State . Town . Street . HouseNumber.
So rather that having to remember the address 192.168.123.132 we POINT a domain name to it like WWW.DOMAIN.COM. Also you can have FTP.domain.com go to a different machine or address and so on.
As you can see, a lot of what machines do is based on hierarchy. The IP address builds from left to right, a domain name builds from right to left. There is also a hierarchy in how the domain names are managed. The machines that do this are called Domain Name Servers or DNS's. The ROOT DNS's manage simply the high level domains .com .org .net etc, and they POINT to another DNS(the primary DNS) that has specifics about that domain. DNS not only GIVES information, but they find it too. Your computer is configured to talk to a DNS supplied by your provider (Verizon, Comcast) to help you find things on the net. So it works like this, you type in WWW.GOOGLE.COM, your Comcast DNS says I don't know him, it the goes to the ROOT DNS of .COM and says, can you tell me who the primary is? ROOT replies yes the PRIMARY DNS for google.com is 192.168.123.132, your Comcast DNS then goes to the PRIMARY 192.168.123.132 and says, can you give me the address for WWW.google.com? Primary replies WWW is 10.123.132.169. The Comcast DNS then gives your machine the address 10.123.132.169 to get the web page. Bet you never thought all that was going on did you!
To reduce all of this traffic, a DNS caches, or remembers who he has asked about. So now if someone else asks the Comcast DNS where is www.google.com, he remembers it and does not have to ask root. A DNS only remembers for a specific amount of time, from 24 to 1 hour. In the past it was a week, and this caused problems, because if I moved www.google to another server, thus another address. But the Comcast server would still be giving out the old address for 7 days, so now most are 24 hours. (its called TTL or Time To Live)