Sub domain

Lets say your domain is http://www.mysite.com.
You install a Message Board and put in in a directory called http://www.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 http://www.mysite.com/board OR http://www.board.mysite.com


Parked Domain

You have two domains http://www.mysite.com and http://www.my-other-site.com.
http://www.mysite.com is the domain of your website and you want to add http://www.my-other-site.com.
You want them both to go to the same place. In other words, when someone types either http://www.mysite.com or http://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 http://www.mysite.com and http://www.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).