A subdomain is a web address, which is 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 single subdomain can have its own website and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be completely independent and also hosted by a separate company. Using a subdomain can be very practical if you wish to have different sites that are related in some way. For example, you could have a web-based store under the main domain as well as a forum under a subdomain so as to offer means for your clients to ask questions or share views regarding your products. The best thing about using 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.