A subdomain is a web address, that is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a separate provider. Using a subdomain can be very useful if you wish to have different websites which are associated in some way. For instance, you can have an online store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and share views regarding your products. The benefit of using a subdomain instead of a subfolder is that the web sites will be separated, so you can update one of them or perform maintenance not affecting the other.