A subdomain is a website address, that is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain name, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. apart from employing the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate company. Using a subdomain could be very handy if you wish to have different websites which are related in some way. For instance, you can have an online store under the main domain plus a forum under a subdomain so as to provide means for your customers to ask questions and / or share thoughts concerning your products. The main advantage of employing a subdomain instead of a subfolder is that the sites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.