A subdomain is a website address, that is under a main, or a root domain. An illustration is something.domain.com, where domain.com is the main domain, 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 also hosted by a separate provider. Using a subdomain can be very beneficial if you want to have different web sites which are related in some way. For example, you can have a web-based store under the main domain along with a forum under a subdomain in order to offer means for your customers to ask questions and / or share thoughts regarding your products. The advantage of employing a subdomain instead of a subfolder is that the web sites will be separated, so you are able to update one of them or perform maintenance without affecting the other.