What is a Subdomain?
The subdomain is the "www" in a website's URL, or any text code in its place. For example, in the fully-qualified domain name (FQDN) www.reddit.com, the www is the subdomain, while reddit is the second-level domain, and com is the top level domain (TLD). In old.reddit.com (which accesses Reddit's old design), the text code old is the subdomain.
It's possible to have several subdomains in sequence, e.g. a.b.c.d.example.com.
When you buy a domain name, you pay a yearly fee for something like example.com, and you can configure as many subdomains as you wish. Most websites only configure the www subdomain, leading to web browsers like Chrome hiding the www in the address bar.
Some websites use subdomains to host translated versions of the website. As of writing, this website hosts its Portuguese version in its br subdomain: br.virtualcuriosities.com. That's because com and com.br are different TLDs. This means I would need to pay two different yearly fees if I wanted a com.br version of my website, when I could just use the br subdomain of the domain I already pay for for free. Large websites like Google and Facebook actually own several TLD variations of their domains, e.g. facebook.com.br and google.com.br. Most websites just use a directory for their translations instead, e.g. example.com/pt-br/.
Some websites are hosted on other websites, accessible as subdomains of their hosts. For example, inkscapetutorials.wordpress.com is hosted on wordpress.com, a company that offer WordPress hosting. It's common practice for web hosting services to make websites of their customers available as subdomains. When you purchase a domain name, you simply configure your www subdomain to be associated with the FQDN you received from your host (e.g. inkscapetutorials.wordpress.com), and that's all the configuration you need to make www work.
Some other examples include:
*.blogspot.com- Google's Blogger platform.*.tumblr.com- Tumblr, a blogging platform trying to become social media.*.neocities.org- Neocities, free static website hosting.