When you own a website, it comes with a unique address like “WordPress.org” that no other website in the world can use. This is also called your domain name.
When you first install WordPress on a specific URL, everything will be running on that domain. However, it is possible to split up different parts of your WordPress site into different “addresses,” all of which are “under” or part of the domain. Therefore, some people refer to the first domain as the “top domain” or “top-level domain.”
Just as you might expect, a subdomain is a smaller subset domain underneath the main domain. The subdomain acts like a completely separate website, but it is still hosted at the same “place” on the internet.
For a simple blog, you probably won’t need a subdomain, but more complex websites can benefit from subdomains in order to keep things more organized. For instance, if you have a top-level domain called “www.MyAwesomeBlog.com” that’s free for the general public to use, you could set up “VIP.MyAwesomeblog.com” as a subdomain for paying customers that allows them to access exclusive content.
The most common use of a subdomain is for e-commerce websites. The catalog of products usually exists on the top-level domain, but the payment part exists on a subdomain because it requires more sophisticated settings in order to transfer payment information securely.
Another common use of subdomains that is being phased out is when you have a mobile version of your website that’s different than the one for desktop computers.
In previous years, “www.MyAwesomeBlog.com” would have been designed for full-screen desktop computers while “m.MyAwesomeBlog.com” would have been optimized for mobile devices. This is no longer recommended, however, as advances in adaptive programming allow top-level domains to automatically adjust to the user’s computer or device.