A subdomain is a website url, that is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each and every subdomain can have its own website and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be completely independent and also hosted by a separate company. Using a subdomain can be quite useful if you want to have different sites that are associated in some way. As an example, you can have a web-based store under the main domain as well as a forum under a subdomain so as to supply means for your customers to ask questions and / or share views regarding your products. The benefit of using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you can update one of them or perform maintenance not affecting the other.