If your plan allows to lodge additional domains, you will be able to lodge in your account domains in addition to the main one. Your visitors will see them totally independent, although you will manage them from the same Control Panel cPanel of the main domain of your account of Hosting, and will consume the resources of your plan of Hosting.
In order to add additional domains, cPanel of the account of Hosting accedes to the Control Panel in which it is going to add the domain, additional section Domains, Domains. We see an example exceeds how to fill up the data:
New Name of Domain: nuevodominio.com
Usuary Subdominio/FTP: nuevodom
Root document: public_html/nuevodominio.com
You will see that it creates a new folder to you in your account with the name of the subdomain, nuevodom in this case, is there where you must put the files of the new domain that you finish creating. We say that it is an account within another account, but sharing the same Control Panel.
NOTE: Before adding it you must have formed that domain so that it uses the same servers DNS or nameservers that you are using in your main domain.