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

Parking
inetservices.net —–(points to)—-> inetservices.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 a secure shopping cart. See the examples below.

Subdomain
secure.yourdomain.com
myweblog.yourdomain.com
xyz.yourdomain.com

A parked domain does not create any subdirectories on your account. A subdomain will create a subdirectory in your public_html directory. The subdirectory name will be the name of the subdomain unless you change it in your account’s cPanel when adding the subdirectory.