A subdomain is a web address, that's under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside 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. Employing a subdomain could be very useful if you'd like to have different sites which are connected in some way. For example, you may have an online store under the main domain plus a forum under a subdomain so as to offer means for your clients to ask questions and share experiences concerning your products. The best thing about using a subdomain as opposed to a subfolder is that the web sites will be separated, so you are able to update one of them or perform maintenance without affecting the other.