A Domain is a part of the Domain Name Service (DNS) name hierarchy that represents a collection of DNS resource records of computers and resources hosts that share part of their fully qualified name and whose DNS information is managed as a group.
A domain is a concept, represented by a zone file in a DNS server's configuration. It is a useful concept for discussing DNS issues. A zone file holds the contents of a DNS domain.
The term domain is often used incorrectly to refer to the actual name of a domain.
How does this work?
Lets pretend we have four computers whose names are four, three, two and one. Lets pretend all four hosts belong to a single domain named beers-on-the-wall.org.
Their fully qualified domain names would be four.beers-on-the-wall.org, three.beers-on-the-wall.org, two.beers-on-the-wall.org, one.beers-on-the-wall.org
The domain contains all the hosts in beers-on-the-wall.org.
[pfew! Thank goodness for cut and paste! --InetD ]