A subdomain is a website url, that is under a main, or a root domain name. One example 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. apart from using the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a different provider. Using a subdomain can be extremely useful if you wish to have different websites that are related in some way. As an example, you may have an on-line store under the main domain and a forum under a subdomain so as to offer means for your clients to ask questions and / or share views about your products. The main advantage of employing a subdomain rather than a subfolder is that the web sites will be separated, so you are able to update one of them or perform maintenance not affecting the other.