A subdomain is a web address, that is under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be completely independent and even hosted by a separate company. Employing a subdomain can be quite handy if you'd like to have different web sites that are connected in some way. For instance, you could have an online store under the main domain as well as a forum under a subdomain so as to offer means for your customers to ask questions or share opinions concerning your products. The benefit of using a subdomain rather than a subfolder is that the web sites are going to be separated, so you can update one of them or perform maintenance not affecting the other.