A subdomain is a web address, that is under a main, or a root domain name. An illustration 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 employing the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a separate company. Using a subdomain could be very useful if you wish to have different sites which are related in some way. For instance, you could have a web-based store under the main domain and a forum under a subdomain so as to offer means for your customers to ask questions and / or share views regarding your products. The benefit of employing a subdomain instead of a subfolder is that the web sites will be separated, so that you can update one of them or perform maintenance not affecting the other.