Subdomains are a way of creating separate accounts within your main account.
The subdomain can be accessed through a separate URL in the form of 'http://subdomain.domainname'.
This represents a subfolder within the public_html level of your account and you can create e-mail accounts with the subdomain extension.
Subdomains are often used to make access to subdirectories easier by simply typing in a shorter domain name, rather than the full domain name and the directory name.
Domain pointers, parked domains, domain alias or domain forwarders:
This allows you to point multiple domain names to the same website.
For example: yourdomain.com, yourdomain.net and somethingelse.com can all point to the same location. However, it CANNOT point to a folder on the main domain and it does not have any FTP features.
Domain pointers are free; however, you need to pay for the domain registration and you must point them to the same nameservers of your main domain.
An add-on domain is a secondary domain name that points to a folder (subdirectory) within the main hosting account. It can be added from your cPanel.
www.seconddomain.com will show the files actually in www.maindomain.com/afolder/
An addon domain will act like a full account; you can create sub-domains, email accounts, and FTP accounts for your addon domains directly from your cPanel .
Notice that the price listed for add-on domains does NOT include domain name registration fees. You need to pay for the domain registration and you must point them to the same nameservers of your main domain.