A subdomain is a website url, that is 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. apart from using the domain name as a part of the whole web address, a subdomain can be totally independent and also hosted by a different company. Employing a subdomain could be very beneficial if you wish to have different websites which are related in some way. For instance, you may 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 using a subdomain rather than a subfolder is that the web sites will be separated, so that you can update one of them or perform maintenance not affecting the other.