A subdomain is a web address, that is under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from employing the domain name as a part of the whole web address, a subdomain can be completely independent and also hosted by a separate service provider. Using a subdomain can be very useful if you'd like to have different web sites which are related in some way. As an example, you can have a web-based store under the main domain plus a forum under a subdomain in order to supply means for your customers to ask questions or share opinions about your products. The advantage of employing a subdomain rather than a subfolder is that the web sites are going to be separated, so that you can update one of them or perform maintenance not affecting the other.