Briefly, a subdomain is the additional part of the primary domain. Therefore, a sub-domain or a third level domain is the third part of a domain name. Thus, the search engines considered the sub-domain as a separate website. For example – Here, the blog is a subdomain and although is a domain name, templatetoaster is considered as a subdomain of .com (Top Level Domain). It also means that the search engines crawl or index through every subdomain separately. 

Many organizations use the sub-domain to assign a unique name to particular departments. Often used by internet service providers many supplying web services also use the subdomains. They allocate one (or more) subdomains to their clients who do not have their own domain name. It enables you to set up multiple websites using a single domain.

Moreover, creating a testing version of the website also uses subdomains. Mostly, the developers test the new plugins on subdomain staging websites, before making them live. In like manner, sub-domains are a major part of eCommerce websites. Mostly, online stores look for another subdomain to handle payments and transactions. Besides, you can also install a sub-domain on your website to host a specific group like guest users, registered users and more. Additionally, the sub-domains can be very useful in helping you categorise the content on the website.