A subdomain is a web address, which is under a main, or a root domain. One example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a different provider. Using a subdomain can be extremely practical if you want to have different web sites that are related in some way. As an example, you could have a web-based store under the main domain as well as a forum under a subdomain so as to offer means for your customers to ask questions and share views concerning your products. The advantage of employing a subdomain instead of 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.