A subdomain is a website url, which is under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. apart from using the domain name as part of the whole web address, a subdomain can be totally independent and even hosted by a separate service provider. Employing a subdomain can be very practical if you wish to have different websites that are associated in some way. For example, you may 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 experiences regarding your products. The benefit of using a subdomain rather than a subfolder is that the websites will be separated, so you're able to update one of them or perform maintenance without affecting the other.