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 address, 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 separate service provider. Using a subdomain can be quite practical if you'd like to have different websites that are associated in some way. For instance, you may have a web-based store under the main domain along with a forum under a subdomain in order to provide means for your clients to ask questions and / or share thoughts regarding 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.