A subdomain is a web address, that's under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a different service provider. Using a subdomain can be quite handy if you wish to have different websites that are related in some way. For instance, you may have an on-line store under the main domain along with a forum under a subdomain so as to provide means for your clients to ask questions or share thoughts regarding your products. The main advantage of using a subdomain rather than a subfolder is that the websites are going to be separated, so you are able to update one of them or perform maintenance without affecting the other.