Subdomain
Subdomain refers an address for an independent website, a website with its own files away from the main or the primary domain files. Sub-domains are usually named as a subset if the primary domain i.e.
Subdomain.primarydomain.com
It usually replaces www within the main domain.
Remember main/primary domain stores its files within the www directory.
Subdomain is usually used when a website has other microsites or sites to be hosted within the primary domain without buying a new domain.
Add-on Domain
Add-on Domain refers to a domain added to an existing hosting account. With an add-on domain, one can create email address for it, install scripts, get analytics among other domain related functions; however, addon domain does not have its own Cpanel but shares with the main/primary domain.
Files and directories of an add-on domain are treated as a subdomain to the main/primary domain.
i.e. http://addondomain.primarydomain.com
(Sub) Directory – http://primarydomain.com/addondomain/
Parked Domain
Refer to a domain placed in the hosting account of the main or primary domain just to redirect to the main domain.
All the traffic on the parked domain is redirected to the main or primary domain.
It acts as an alias of the main/primary domain.
Usually utilized when one as different domain extensions for the same domain.
Can also be utilized if there is possible misspelling mistake from the web visitors or if one misspelled a domain during domain registration, the misspelled domain may be parked to the correct domain.