An Alias domain does not have a unique website. It is a domain that is masked to forward to a primary domain of the cPanel account. For example, an alias could be a common misspelt version of your main website URL address which would then be forwarded automatically to your main website. Another use is for email; a parked domain can provide a separate email account.
An addon domain acts as a second website with it's own unique design and pages. Many cPanel accounts, including ours, offer addon domains - for example if 5 were available in a hosting package, in theory you can hold 6 websites on the same account which is cost effective.
Please note - you would need to own separate domains to be able to set up addon domains.
A subdomain acts as a second website with its own unique design and pages. However, unlike an addon domain, you would not need to own a separate domain. If your website was www.your-domain.com, you can replace the www. with a subdomain name of your choice such as blog.your-domain.com.