All 3 are included with your hosting account. Depending upon which server you are hosted on, setup is either automated or manual.
A subdomain is a second website, with its own unique content, but there is no new domain name. Instead, you use an existing domain name and add a period preceeding your domain and the name of your sub-domain. The subdomain name looks like forums.domain.com, help.domain.com, help2.domain.com (assuming you already host domain.com).
An addon domain is a second website, with its own unique content. This type does require you to register the new domain name before you can host it. So, if you want to host www.otherdomain.com, this is the solution. NOTE: You can have multiple sites within your one account, however they all share the same total web space. If you have a 1 gig web plan, all sites combined cannot exceed 1 gig. If they do, you'll need to increase to a larger plan.
A parked domain is NOT a unique website. Instead, it is a masked forward to the primary domain name of your account, much like a website alias. Parked domains are commonly used when you need a place to park a domain you do not have a website for, when you have more than one domain that should lead to your primary domain, or when you have common misspellings of your domain name that you have registered.
