A subdomain is a website url, that's under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Every single subdomain can have its own site and records i.e. aside from using the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a separate company. Using a subdomain could be very practical if you wish to have different web sites that are related in some way. For instance, you may have an online store under the main domain along with a forum under a subdomain in order to supply means for your customers to ask questions and / or share opinions concerning your products. The main advantage of employing a subdomain rather than a subfolder is that the websites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.