A subdomain is a website address, that is under a main, or a root domain. An 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 website and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a separate service provider. Employing a subdomain can be extremely useful if you want to have different web sites that are connected in some way. For instance, you could have an on-line store under the main domain along with a forum under a subdomain in order to provide means for your customers to ask questions and / or share thoughts about your products. The benefit of employing a subdomain instead of a subfolder is that the sites will be separated, so you are able to update one of them or perform maintenance not affecting the other.