Sub-Domain Name

The Sub-Domain Name is your address on the Internet. Its existence relies on the Domain of which it is a part. There is no need for this to be registered with an outside body, so is cheaper.

Sample:
www.cub.rv.com.au could be the sub-domain name of Cub Camper Trailers
The '.rv' indicates the Domain
The '.com' indicates it is a commercial site, others include .net,.org,.gov
The '.au' indicates it is registered in Australia

A Sub-domain name is generally FREE on www.rv.com.au however some names have been reserved and are available for a annual fee.

Warning:
You should be aware, that being a sub-domain carries a certain risk. If the domain owner either stops providing the service, or overprices it, any advertising of your Sub-Domain name, and any 'Search Engine' index will be wasted.

There are no rules for Sub-Domain names. However:

They should not be too long
They should be easy to remember
Don't use commonly misspelt words
They must not be offensive