A subdomain is the part of the web address which is before a domain and you have most likely seen a lot of subdomains while browsing world wide web. As an example, many websites like Wikipedia have versions in several languages using subdomains - en.wikipedia.org, de.wikipedia.org and so forth. The advantage of employing a subdomain is that it can have a separate site and its own records, so you're able to even host it on a different server. The practical use is that you could have a supplementary site, such as an e-learning portal for pupils on top of the primary school website. If you work with subdomains as opposed to subfolders, it's going to be much easier to perform maintenance or to upgrade a certain website, not mentioning that it will be more safe to have the websites separate from one another.