A subdomain is a website url, that's under a main, or a root domain. An example is something.domain.com, where domain.com is the main domain name, 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 part of the whole web address, a subdomain can be completely independent and also hosted by a different provider. Using a subdomain can be extremely useful if you want to have different web sites that are connected in some way. As an example, you may have a web-based store under the main domain as well as a forum under a subdomain so as to provide means for your clients to ask questions and share thoughts about your products. The best thing about employing a subdomain rather than a subfolder is that the websites are going to be separated, so you can update one of them or perform maintenance without affecting the other.