A subdomain is a website url, which is under a main, or a root domain. An illustration 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. apart from using the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a different company. Employing a subdomain could be very useful if you wish to have different sites which are connected in some way. As an example, you can have an online store under the main domain and a forum under a subdomain so as to supply means for your clients to ask questions or share experiences about your products. The benefit of employing a subdomain rather than 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.