A subdomain is a web address, that's under a main, or a root domain. One example 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. aside 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. Using a subdomain could be very practical if you wish to have different websites that are related in some way. For instance, you could have an on-line store under the main domain along with a forum under a subdomain in order to provide means for your customers to ask questions and share views regarding your products. The best thing about using a subdomain as opposed to a subfolder is that the websites are going to be separated, so that you can update one of them or perform maintenance not affecting the other.