A Parked domain is a domain name that leads to the same website as another domain name. The domain is, in essence, parked on top of the other domain.
For example, suppose we have a domain name mysite.com. If we park mysite.org on top of the .com domain, a visitor to our site will be able to access the site from either the .com or the .org address. The visitor won't notice any difference in the website at all no matter which one she chooses.
An Addon domain, on the other hand, is a domain that will display a completely different website. So, If you pay to be able to host an addon domain on your hosting account, you are basically being given permission to host another site in addition to your main website.
Suppose you have MyCatSite.com, but you also want to launch a sister site called MyDogSite.com. You would do this using an addon domain.
It is important to note that buying addon domain or parked domain rights from your hosting provider doesn't actually give you a domain name; it just gives you the ability to add that domain to your account. You'll still have to pay for the domain name, too.
Subdomains, though, will work off of your existing domain names. They can be used for the same purposes as regular domain names; they just have an extra part in front.
www is actually considered a subdomain, but one that is usually parked on top of the main domain.
Another example could be if you created two subdomains dogs and cats on mypetsite.com. You'd visit those subdomains in your browser using http://dogs.mypetsite.com/and http://cats.mypetsite.com/, respectively.
You could set each subdomain to go to its own website, or to go to a section of the main mypetsite.com site.
This is the end of the tutorial. We hope you now completely understand the differences between addon, parked, and sub domains.