A subdomain is a website url, that is under a main, or a root domain name. An illustration is something.domain.com, in which domain.com is the main domain, while "something" is the subdomain below it. Every subdomain can have its own website and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be completely independent and even hosted by a separate company. Employing a subdomain could be very practical if you'd like to have different sites which are associated in some way. For example, you may have an online store under the main domain along with a forum under a subdomain in order to supply means for your clients to ask questions or share experiences regarding your products. The best thing about using a subdomain rather than a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.