A subdomain is a website address, which is under a main, or a root domain name. An example is something.domain.com, in which domain.com is the main domain name, while "something" is the subdomain below it. Each and every subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be totally independent and also hosted by a different company. Using a subdomain can be very handy if you'd like to have different websites that are related in some way. As an 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 and share experiences regarding your products. The advantage of using a subdomain rather than a subfolder is that the websites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.