A subdomain is a website url, that's under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Each 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 fully independent and even hosted by a different provider. Using a subdomain can be quite practical if you want to have different websites which 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 opinions concerning your products. The main advantage of using a subdomain as opposed to 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.