People are familiar with URLs, like this one…
This URL can be broken into distinct parts. https:// is a ‘protocol’, it tells the web browser to load the webpage over a secure connection. /my-page is a specific page on this website. If we remove these parts we are left with;
This part of the URL can be broken up into further parts based on where the full stops are. Each of these parts are then known as ‘domains’. The one on the far right (com) is called the ‘Top Level Domain’, then the next one (example) is commonly referred to as the domain and finally the one on the left (www) is known as the subdomain.
‘www’ is a very common subdomain, but it is completely arbitrary as to whether a website should use it or not. If you ran a website with a blog you might decide that the main content of the site should be www.example.com and the blog should be found at blog.example.com (the ‘blog’ subdomain’). Or you might decide that the main content should just be on example.com and the blog should be on example.com/blog (i.e. no subdomains). Either of these ‘URL structures’ would be valid.
It’s actually a bit more complicated than that
The above URLs were chosen in order to convey common terminology in a deliberately simple way. But reality there can be any number of domains chained together in a URL. For example the following has 4 domains…
In this instance technically uk is the top level domain, uk is the ‘second level domain’, bbc is the third level domain and www is the fourth level domain. It would be common practice to still call www the subdomain however.