A subdomain is a website address, that is under a main, or a root domain name. An 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 website and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be fully independent and also hosted by a separate provider. Using a subdomain can be very handy if you want to have different websites that are connected in some way. As an example, you can have an on-line store under the main domain plus a forum under a subdomain so as to provide means for your clients to ask questions or share opinions concerning your products. The benefit of using a subdomain as opposed to a subfolder is that the sites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.