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. Every 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 totally independent and even hosted by a different provider. Using a subdomain could be very useful if you'd like to have different web sites which are connected in some way. For example, you could have an online store under the main domain plus a forum under a subdomain in order to supply means for your customers to ask questions or share thoughts regarding your products. The benefit of employing a subdomain rather than 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.