Expertise level: Easy
Parked domains have been configured to point towards your server, but are presently unused. WHM will automatically redirect those domains towards other domains of your choice.
Subdomains act as a whole new website, albeit one that shares the resources of the main account.
You can view a list of your parked domains and subdomains in the "Account Information" section. Log into WHM as root and simply search for "Account Information" in the search box to the top left and you will see "List Parked Domains" or "List Subdomains".