A subdomain is a web address, that's under a main, or a root domain. One example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate service provider. Using a subdomain can be quite practical if you'd like to have different web sites which are connected in some way. For example, you can have a web-based store under the main domain plus a forum under a subdomain in order to provide means for your clients to ask questions and share experiences regarding your products. The main advantage of using a subdomain instead of a subfolder is that the websites will be separated, so you are able to update one of them or perform maintenance not affecting the other.