A subdomain is a website url, that's under a main, or a root domain. One example 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. apart from employing the domain name as a part of the whole web address, a subdomain can be entirely independent and even hosted by a different provider. Employing a subdomain can be very practical if you want to have different sites which are associated in some way. For instance, you could have a web-based store under the main domain plus a forum under a subdomain so as to offer means for your customers to ask questions and share views about 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 not affecting the other.