A subdomain is a website url, that's under a main, or a root domain. 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 site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a separate company. Employing a subdomain can be extremely useful if you want to have different sites that are related in some way. For instance, you may have an on-line store under the main domain and a forum under a subdomain in order to supply means for your clients to ask questions and / or share thoughts concerning your products. The best thing about 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 without affecting the other.