A subdomain is a website url, that is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate service provider. Employing a subdomain can be extremely beneficial if you wish to have different web sites which are associated in some way. For example, you could have an on-line store under the main domain as well as a forum under a subdomain so as to offer means for your clients to ask questions or share views regarding your products. The advantage of employing a subdomain rather than a subfolder is that the websites will be separated, so you can update one of them or perform maintenance without affecting the other.