A subdomain is a web address, that's under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a different service provider. Using a subdomain can be quite practical if you'd like to have different web sites which are associated in some way. For example, you could have an on-line store under the main domain along with a forum under a subdomain in order to offer means for your clients to ask questions and / or share experiences regarding your products. The main advantage of using a subdomain instead of a subfolder is that the sites will be separated, so you can update one of them or perform maintenance not affecting the other.