You can add additional domains to your account that will use and share the same disk space and bandwidth of your account. You can add this additional domain from your control panel.
Once added, you can have all the services for this domain as if it was an independent account.
o You can create an ftp account (which is created automatically if you add the domain yourself from control panel) which has restricted access to a sub-folder under public_html.
o You can create email accounts etc.
The only thing that it does not have is separate cpanel account. Your main cpanel user/pass remains the same.
If you need a separate cpanel user/pass for a domain, then you need to signup for a separate account or go with a reseller plan.
Tutorial: How to create addon domain in cpanel.