Domain parking is pointing one domain to another so it loads the same content.
This is generally done if you have many different TDL(s) of the same domain name (mydomain.com, mydomain.net, mydomain.org) parked on top of the main domain for example mydomain.com.
mydomain.net -> mydomain.com
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.