A parked domain is a web site address that points to your current web site. For example if you have the address www.mybusiness.com and would like to use www.mybusiness.com.au as well, you would buy the second web site name and park it on your hosting account, so that your customers could type either and still find you.
Add on domains mean that you can use your hosting account to host more than one web site if you choose. You pay once, but you can use it up to 5 times depending on the plan you select.
The Standard plan is better for the mid sized business or business that intend to setup separate web sites for key products.
You can host up to three web sites with the standard plan and the storage and bandwidth allowances are more than sufficient to handle even the busiest of domains.