A subdomain is a website address, that is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each and every subdomain can have its own website and records i.e. aside from employing the domain name as a part of the whole web address, a subdomain can be entirely independent and also hosted by a separate company. Using a subdomain can be quite useful if you wish to have different web sites which are related in some way. As an example, you can have an online store under the main domain plus a forum under a subdomain in order to offer means for your customers to ask questions and / or share views regarding your products. The advantage of using a subdomain instead of a subfolder is that the websites will be separated, so you can update one of them or perform maintenance without affecting the other.