A subdomain is a web address, that's under a main, or a root domain. An illustration is something.domain.com, where 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 part of the whole web address, a subdomain can be entirely independent and also hosted by a different service provider. Using a subdomain can be extremely beneficial if you want to have different web sites that are connected 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 offer means for your customers to ask questions and share experiences about your products. The best thing about employing a subdomain as opposed to a subfolder is that the sites are going to be separated, so you can update one of them or perform maintenance not affecting the other.