What are the subdomains?

A subdomain, or a third level domain. It’s a great way to create a unique section of your site.
A subdomain combines a unique identifier with a domain name to become essentially a domain within a domain.
The unique identifier simply replaces the www in the web address. WorldPosta, for example, uses subdomains such as mail.worldPosta.com and admin.worldposta.com to reference its mail and music services, under the umbrella of www.worldposta.com.
Below is the model of the structure of a subdomain.

http://

subdomain

.

domain

.

com

3rd level

2nd level

1st level