A subdomain is an extension of your main or addon domain that will be in the following format: subdomain.maindomain.com
Usually you put the subdomains in a separate folder on the documents root (www or public_html), for example www.maindomain.com/subdomain If you want to make your user’s lifes easier, you will simply create a subdomain in the format subdomain.maindomain.com
This is a straightforward and free process since most of the modern hosting packages offer unlimited subdomains