A subdomain is a web address, that's under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a different provider. Employing a subdomain can be quite beneficial if you want to have different sites which are connected in some way. For instance, you can have a web-based store under the main domain and a forum under a subdomain so as to offer means for your clients to ask questions or share experiences regarding your products. The benefit of using a subdomain instead of a subfolder is that the websites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.