This blog post will give you a a better meaning of the domain options in your control panel. This will not explain how to add domains (Sub, Addon, Parked) but here are the links to the blog articles that do.
Adding a Subdomain:http://blog.arvixe.com/how-to-add-sub-domains-to-your-website-using-cpanel/
Adding a Parked Domain:http://blog.arvixe.com/how-to-add-parked-domains-to-your-website-using-cpanel/
Adding an Addon Domain:http://blog.arvixe.com/how-to-add-addon-domains-to-your-website-using-cpanel/
When logging into your control panel you will see the “Domains” box with up to 6 options in it. The only options that this article will cover are the first 3 icons.
Addon domain: An Addon domain can be any domain name: Arvixe.com or billing.arvixe.com. It is not restricted to only domain names, it can also be sub domains. If you do add a subdomain in this section it will count towards your total number of addon domains. This is usually used for making separate sites on the same control panel.
Subdomain: A Sub-domain is any domain that is independent of the main domain and has an extension before the domain name. Subdomains can only be added if the domain is already added to the cPanel. This is usually used for having a site that relates to your main domain but you do not want it in a folder on the main domain (domain.com/files) For example: blog.arvixe.com or support.arvixe.com
Parked Domain: This can be a domain or a sub domain (i.e. same as an addon domain) that points to your main domain. The domain is pointed to the root folder of that domain. The use for this option is for when you have multiple variations of a domain name like .net, .org and you want them to point to the same site, the .com version. When you visit it you are not redirected to the main domain but instead view it as if that was the domain you were visiting. (Note: If your script has a redirected then you will be redirected to the main domain).
These were the are the 3 options for adding a domain/subdomain to your cPanel. With each of these options you also have the ability to redirect the domain to another location. If you need to you can do so by clicking on the “Manage Redirection” next to that Addon domain, Subdomain or Parked domain.