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 website and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be entirely independent and also hosted by a different company. Employing a subdomain could be very useful if you wish to have different websites that are connected in some way. For instance, you could have a web-based store under the main domain along with a forum under a subdomain in order to offer means for your clients to ask questions and share opinions about your products. The main advantage of employing a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance not affecting the other.