A subdomain is a web address, that's 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. Each subdomain can have its own site and records i.e. aside from employing the domain name as a part of the whole web address, a subdomain can be fully independent and also hosted by a different company. Using a subdomain can be quite practical if you'd like to have different web sites that are connected in some way. For example, you could have an online store under the main domain plus a forum under a subdomain in order to offer means for your clients to ask questions and share opinions 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.