(Please read “What is a Domain Name?” and “What are Top Level Domains (TLD)?” first.)
A domain name is made up of a top level domain on the right and the second level domain to the left of it separated by a dot. People and organizations register domain names for periods of a year or more by paying a small annual fee.
The registered user of a domain name can have any number of sub domains under that domain. These sub domains are located to the left of the secondary domain and separated by a dot. For example, the registered owner of the domain netflint.com can point www.netflint.com to a web page, ftp.netflint.com to a file server, and mail.netflint.com to an email server.
Examples of Sub Domains