A subdomain is a website url, that is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a separate provider. Employing a subdomain can be quite useful if you'd like to have different websites that are related in some way. For instance, you could have an online store under the main domain as well as a forum under a subdomain in order to provide means for your clients to ask questions and share thoughts concerning your products. The advantage 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.