There are three types of site you can have in addition to the main account:
Parked Domains - where the parked domain shows the same content as the main account domain (also known as an alias). For Parked Domains, go to Domains -> Parked Domains in the control panel, and add the domain(s) you wish to display the same content as the main domain.
Addon Domains - where the add-on domain shows different content than the main account domain, served from a separate folder. For Add-on Domains, go to Domains -> Add-on Domains in the control panel, and add the additional domain(s) you wish to display a different website under. For each one of the Add-on domains, you will be prompted to create a folder that is associated with the domain name, where you will upload the website files. The new folders will be created within the main site folder. You can also create a specific FTP account for each domain. This account will have access only to the folder defined as the Add-on domain folder, and its subfolders.
Subdomains - which can be subdomains of any domains associated with your account, and they can either be entirely separate sites or show specific content from the main site through redirects.