A subdomain is a website url, that is under a main, or a root domain. One example is something.domain.com, where domain.com is the main domain, while "something" is the subdomain below it. Every 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 entirely independent and even hosted by a separate company. Using a subdomain can be extremely beneficial if you want to have different websites which are connected in some way. For instance, you may have a web-based store under the main domain and a forum under a subdomain so as to offer means for your clients to ask questions and / or share views regarding your products. The best thing about employing 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.