A subdomain is a web address, that is 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 website 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. Employing a subdomain can be quite beneficial if you want to have different sites which are associated in some way. For instance, you could have a web-based store under the main domain along with a forum under a subdomain in order to provide means for your customers to ask questions or share opinions about your products. The best thing about employing a subdomain instead of a subfolder is that the web sites are going to be separated, so you can update one of them or perform maintenance without affecting the other.