A subdomain is a web address, that's under a main, or a root domain name. One example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate provider. Employing a subdomain could be very practical if you'd like to have different web sites that are related in some way. For example, you can have a web-based store under the main domain and a forum under a subdomain so as to provide means for your customers to ask questions and / or share thoughts about your products. The benefit of employing a subdomain rather than a subfolder is that the sites will be separated, so you're able to update one of them or perform maintenance without affecting the other.