A subdomain is a website address, that is under a main, or a root domain. 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 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 provider. Employing a subdomain could be very handy if you want to have different websites that are related in some way. For example, you could have an online store under the main domain and a forum under a subdomain in order to provide means for your clients to ask questions or share experiences about your products. The best thing about using a subdomain instead of a subfolder is that the sites are going to be separated, so you can update one of them or perform maintenance without affecting the other.