A subdomain is a web address, that is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be completely independent and also hosted by a different company. Employing a subdomain can be very useful if you wish to have different web sites which are associated in some way. As an example, you could have an online store under the main domain plus a forum under a subdomain so as to supply means for your customers to ask questions and share opinions about your products. The best thing about using a subdomain instead of a subfolder is that the websites will be separated, so you're able to update one of them or perform maintenance without affecting the other.