A subdomain is a website url, which is under a main, or a root domain name. An example is something.domain.com, in which 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. aside from employing the domain name as 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 very beneficial if you wish to have different web sites which are associated in some way. For example, you may have a web-based store under the main domain plus a forum under a subdomain in order to provide means for your customers to ask questions and / or share thoughts concerning your products. The main advantage of employing a subdomain rather than a subfolder is that the sites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.