subdomain is an extension of your main domain name, its position before the domain, like this example: store.host.com. Store.host.com is the subdomain of host.com, some URLs include www, this prefix is also called the subdomain. A subdomain is often used for child-site under the same main domain name or special content that needs to be made separately from the main website like blog, marketing campaign pages, shopping cart pages, etc.
A subdirectory (or subfolder)， in the URL address, the category is positioned after the domain and extensions and is preceded by a slash symbol. It allows you to create a folder that has the same server as the main domain. Unlike subdirectory, subdomain has a different server and it’s independent.