There are three domain types. (Parked, Sub Domains, Add on, )

1. Parked domain: A parked domain is a domain name (eg; example.com) that points to the front page of the main domain. You can not park a domain over a sub.domain or a add-on domain.

2. Sub.domain is when you replace the www. with any word you want. For example a common sub.domain is news.google.com. As you can see there is no www. like most url's instead there is a term used to explain what that locations offers.

3. Add-on domains are the most popular of the three, this powerful tool allows you add top level domain names to any folder in your domain. For example you have a second domain called your-main-domain2.com, you can have that domain point to your-main-domain.com/addonfolder or any other folder you want. It is almost like receiving more web hosting accounts for free.