A parked domain or Alias is an alias/mirror of your primary domain name's website. The website contents will be mirrored, however it will allow you to create separate email addresses.
An add-on domain is similar to a parked domain, however it allows you to specify its own home directory to load the website's contents from. Add-on domains allow you to host multiple domains from your hosting account, which shares all of your allocated resources. An add-on domain also allows you to create email addresses on the domain.
You can create both parked/alias domains and add-on domains by logging into cPanel. Once you have logged in, scroll down to the "Domains" section, where you'll find options to add or remove a Parked/Alias domain or an Addon domain.