A subdomain is a web address, that is under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be fully independent and even hosted by a different service provider. Using a subdomain can be very beneficial if you'd like to have different sites which are connected in some way. For example, you could 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 share opinions concerning your products. The advantage of employing a subdomain rather than a subfolder is that the websites will be separated, so that you can update one of them or perform maintenance not affecting the other.