Say you own two domains: first.com and second.com. First.com is the primary domain for your hosting account - in other words, the domain name you specified when you got your hosting account.
Parked domain:
Go to cPanel ->Parked Domains and park second.com
Result: People type http://www.second.com in their browser, and land at the http://www.first.com home page.
When setting up a parked domain in cPanel, do NOT specify the www in the domain name, just type in "second.com" without the quotes.
Subdomain (or sub domain):
Is just a fancy way of addressing a subdirectory inside your main domain, and does not require you to own a separate domain name other than your main domain. However it is good for search engines because the search engines will see a subdomain as a separate url and will index and list it as a separate web site.
Example: Go to cPanel ->Subdomains. Make a subdomain called "second" (without quotes). A folder called "second" gets automatically created in your web space complete with its own cgi-bin. The url for the subdomain is http://second.first.com You upload your pages into the "second" folder in your first.com web space, using your normal first.com hosting username and password.
Add-on Domain:
Uses a subdomain inside your first.com web space but behaves as a completely separate web site with its own primary domain url.
Example: Go to cPanel ->Add-On Domains. Set up addon domain second.com with the folder/username "second" (without quotes). cPanel automatically creates a folder called "second" in your web space complete with its own cgi-bin. You upload your pages into the "second" folder in your first.com web space using your normal first.com username and password. However, when people type http://www.second.com in their browser, they will see whatever new web site you uploaded into the "second" folder. They will also still of course see the usual first.com site if they type in http://www.first.com
When setting up an addon domain in cPanel, do NOT specify the www in the domain name, just type in "second.com" without the quotes.
For subdomain and addon domain, there cannot be a pre-existing folder of the same name already in your web space. This is because cpanel will create the folder for you and if it finds a pre-existing folder the operation will fail.
IMPORTANT FOR ALL PARKED OR ADD-ON DOMAINS: For a parked or addon domain, you must already also own the second domain name (second.com in our examples) and you must have already set the nameservers for it to the same nameservers as your primary hosted domain uses. The nameservers DNS for both domains must be completely resolved *before* you can attempt to set up the parking or addon in your cpanel. Remember it may take a couple of days after you set the name servers for them to completely resolve.
If you try to set up your parked or addon domain before the DNS resolves, the process will fail. In the case of an addon domain, cPanel will be able to create the folder but the rest of the process will fail. This means that before you can try again you will have to delete the new folder that cpanel created in your web space.
