The Domain Name System (DNS) has a tree structure or hierarchy. Below the TLD's are various other levels:

Second-Level Domain (SLD)
A second-level domain (SLD or 2LD) is the section of a domain name that is to the left of the dot.
For example, in the domain name ‘silvertrees.net’, ‘silvertrees’ is the SLD and ‘.net’ is the TLD. 
The second-level domain is often thought of as the name or identity of the domain where the business, brand, organisation or identifier is displayed.

ccSLD
A ccSLD is a country code second-level domain. 
For example, in the domain name ‘easylandingpages.com.au’ ‘.com’ is the ccSLD (country code second-level domain) and is to the left of the dot 
‘.au’ is the ccTLD and is to the right of the dot.

Subdomains
A subdomain (also known as a child domain) is an additional part to your main domain name under the Domain Name System (DNS) hierarchy.
In the example ’feast.restaurant.com’, ‘feast’ is the subdomain, ’.restaurant’ is the Second-level domain (SLD) and ‘.com’ is the top level domain (TLD). 
Subdomains are created to organise and navigate to specific sections of your website. You can create multiple subdomains on your main domain.