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
change the www to another name. 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. Please read before creating
an Addon Domain.
A parked domain is NOT a unique website.
Instead, it is a masked forward to the primary domain name of your
cPanel 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.
For example, if you own
mydomain.com and it is the primary domain of your cPanel account, you
can set up ourdoman.com as a parked domain and it will load the content
of mydomain.com. You will notice that although the content of
mydomain.com is loading in the browser, the address bar will display
http://ourdomain.com/.