A subdomain is a website address, that is under a main, or a root domain name. 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. aside from employing the domain name as a part of the whole web address, a subdomain can be entirely independent and even hosted by a separate provider. Using a subdomain could be very handy if you want to have different websites which are connected in some way. As an example, you can have an online store under the main domain and a forum under a subdomain so as to provide means for your customers to ask questions and / or share experiences concerning your products. The advantage of employing a subdomain instead of a subfolder is that the websites will be separated, so you can update one of them or perform maintenance without affecting the other.