A subdomain is a website url, that's under a main, or a root domain name. 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 using the domain name as a part of the whole web address, a subdomain can be entirely independent and also hosted by a different company. Using a subdomain can be quite handy if you wish to have different websites that are connected in some way. As an example, you may have an on-line store under the main domain plus a forum under a subdomain so as to supply means for your clients to ask questions and / or share views regarding your products. The best thing about using a subdomain rather than a subfolder is that the sites will be separated, so you're able to update one of them or perform maintenance not affecting the other.