Wikipedia defines domain parking as follows:
"Domain Parking" is the registration of an Internet domain name without using it for services such as e-mail or a website i.e without placing any content on the domain. This may be done to reserve the domain name for future development, to protect against the possibility of cybersquatting, or to engage in cybersquatting. Since the domain name registrar will have set name servers for the domain, the registrar or reseller potentially has use of the domain rather than the final registrant.
When owning multiple TLD's (top level domains eg. .co.za or .com) its often common practice to point the parked domains (mydomain.com, mydomain.net, mydomain.co.za) to the main domain for example mydomain.com.
mydomain.net -> mydomain.co.za
A subdomain is creating another domain off of your main domain name. These are done for different divisions of your company or for having organized separated sections of your site, could be for secure shopping carts, blogs, photo galleries, etc. See the examples below.