A subdomain is a website address, which is under a main, or a root domain. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each and every 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 different company. Using a subdomain can be extremely practical if you'd like to have different sites that are related in some way. For instance, you can have an online store under the main domain along with a forum under a subdomain in order to provide means for your customers to ask questions and / or share thoughts regarding your products. The best thing about employing a subdomain instead of a subfolder is that the web sites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.