A subdomain is a website address, that's under a main, or a root domain name. An illustration 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 website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be entirely independent and even hosted by a separate provider. Employing a subdomain could be very useful if you'd like to have different web sites which are connected in some way. For example, you could have an on-line store under the main domain and a forum under a subdomain in order to offer means for your clients to ask questions and / or share views concerning your products. The best thing about using a subdomain rather than a subfolder is that the sites will be separated, so you are able to update one of them or perform maintenance not affecting the other.