A subdomain is a web address, that's under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. aside from employing the domain name as part of the whole web address, a subdomain can be fully independent and also hosted by a separate company. Using a subdomain can be quite useful if you'd like to have different websites that are associated in some way. For example, you may have a web-based store under the main domain as well as a forum under a subdomain so as to provide means for your customers to ask questions and / or share opinions about your products. The advantage of employing a subdomain rather than a subfolder is that the websites will be separated, so you can update one of them or perform maintenance not affecting the other.