A subdomain is a web address, that is under a main, or a root domain name. 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 website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be totally independent and also hosted by a different company. Using a subdomain can be very useful if you want to have different web sites that are associated in some way. For example, you can have an on-line store under the main domain plus a forum under a subdomain so as to offer means for your clients to ask questions and / or share opinions about your products. The best thing about using a subdomain instead of a subfolder is that the sites will be separated, so you can update one of them or perform maintenance without affecting the other.