A subdomain is a website url, that's under a main, or a root domain. An example is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a separate company. Employing a subdomain could be very practical if you wish to have different sites which are related in some way. As an example, you can have an on-line store under the main domain plus a forum under a subdomain so as to provide means for your clients to ask questions and share opinions about your products. The advantage of employing 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.