A subdomain is a web address, that's under a main, or a root domain. One example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be completely independent and also hosted by a separate company. Using a subdomain can be extremely useful if you'd like to have different web sites that are associated in some way. For example, you can have an on-line store under the main domain as well as a forum under a subdomain so as to provide means for your clients to ask questions or share thoughts regarding your products. The best thing about using a subdomain as opposed to 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.