A subdomain is a web address, that's under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Every single 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 entirely independent and also hosted by a different provider. Using a subdomain could be very handy if you'd like to have different web sites which are connected in some way. For example, you could have an on-line store under the main domain as well as a forum under a subdomain so as to provide means for your clients to ask questions and / or share views concerning your products. The benefit of employing a subdomain instead of a subfolder is that the websites will be separated, so you are able to update one of them or perform maintenance not affecting the other.