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Sub domain

• Lets say your domain is mysite.com.
• You install a Message Board and put in in a directory called mysite.com/board/.
• You can turn the directory board into a sub-domain by adding it as a sub-domain from your Control Panel.
• Now you can access your Message board as either mysite.com/board OR board.mysite.com
• This costs you nothing and is free.

Parked Domain

• You have two domains mysite.com and my-other-site.com.
• mysite.com is the domain of your website and you want to add my-other-site.com.
• You want them both to go to the same place. In other words, when someone types either www.mysite.com or www.my-other-site.com they will go to the same page(s) on your website.
• In order to have additional parked domains you need to purchase the additional domain(s).

Add-On Domains

• You have two domains mysite.com and my-other-site.com.
• You want the two domains to be totally separate/independent websites.
• In order to have additional parked domains you need to purchase the additional domain(s).

source : http://webnet77.com