A subdomain is a website url, which is under a main, or a root domain name. An example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each and every 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 completely independent and also hosted by a separate provider. Using a subdomain can be very practical if you'd like to have different web sites which are associated in some way. For instance, you could have an on-line store under the main domain and a forum under a subdomain so as to provide means for your clients to ask questions and / or share opinions regarding your products. The advantage of employing a subdomain rather than a subfolder is that the sites are going to be separated, so you are able to update one of them or perform maintenance not affecting the other.