Parts of a DNS Website

Example: admin.tryhackme.com

TLD (Top-Level Domain)
tryhackme.com
the TLD is .com.

Two types of TLD
gTLD (Generic Top Level)

• was meant to tell the user the domain name's purpose; for example, a .com would be for commercial purposes**, .org for an organisation.

ccTLD (Country Code Top Level Domain). 

• was used for location like .ca for sites based in Canada.

Second-Level Domain
Taking tryhackme.com as an example, tryhackme is the Second Level Domain.

Subdomain
It's on the left-hand side of Second-Level Domain. For example, in the name admin.tryhackme.com the admin part is the subdomain. 

Multiple subdomains is possible such as jupiter.servers in jupiter.servers.tryhackme.com.

Restrictions when registering domains