A subdomain is a website address, that's 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. Each and every 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 totally independent and even hosted by a separate service provider. Using a subdomain could be very practical if you'd like to have different websites which are related in some way. For example, you could have an on-line store under the main domain along with a forum under a subdomain in order to offer means for your customers to ask questions or share views concerning your products. The best thing about employing a subdomain instead of a subfolder is that the web sites are going to be separated, so that you can update one of them or perform maintenance not affecting the other.