A subdomain is a website url, that's under a main, or a root domain name. An 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. aside from using the domain name as part of the whole web address, a subdomain can be completely independent and even hosted by a separate provider. Employing a subdomain can be very handy if you want to have different web sites that are associated in some way. For example, you could have an on-line store under the main domain and a forum under a subdomain in order to offer means for your clients to ask questions and share thoughts concerning your products. The advantage of using 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.