A subdomain is a website url, which 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. Every single subdomain can have its own site and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a different service provider. 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 and a forum under a subdomain so as to supply means for your customers to ask questions and share views about your products. The best thing about using a subdomain instead of 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.