A subdomain is a domain hosted on its parent domain, much like the parent is a subdomain of the Top Level Domain, such as com, org, and net.
Purpose of Subdomains
Subdomains have served a variety of purposes over the years, a main purpose is the hosting of different services, and this remain one of their uses even today.
Some websites use the www subdomain when hosting their website, while others use the bare domain (I.e. mydomain.com). Some websites might also use subdomains to host multiple versions of their website, such as an English version on the en, and a Danish version on the da subdomain.
How subdomains works
A Subdomain is the first part of the URL, found before the parent domain.