What is Subdomain
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain name. Subdomains are separated by dots “.” and we can access them as sudomain.maindomain.com or domain.com/subdomain and are read from left to right.
Popular real-life examples of subdomains include news.google.com, mail.yahoo.com, in which news, mail and en are subdomains, respectively.
What is Parked domain
Domain pointers allow you to point or park additional domain names to your existing web hosting account. This will allow users to also reach your website when entering the parked or pointed domain into their browsers.
For example: if you park domain102.com at your hosting account for domain101.com, users visiting domain2.com will see the contents of domain1.com web page.
In simple way you can access one website with two different names.
What is Addon domain
Addon domains allow you to host more than one unique website (such that have different content) on your hosting account.
Example: domain101.com is the domain name you have signed up for. Then you decide to host another unique website, reached at domain102.com on the same account. That is when you need an addon domain.