A subdomain is a website address, that is under a main, or a root domain. An illustration is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Every subdomain can have its own site and records i.e. apart from using the domain name as a part of the whole web address, a subdomain can be fully independent and even hosted by a separate company. Employing a subdomain can be extremely useful if you want to have different web sites which are related in some way. For instance, you could have an online store under the main domain and a forum under a subdomain so as to provide means for your clients to ask questions and share opinions regarding your products. The best thing about using a subdomain as opposed to a subfolder is that the web sites are going to be separated, so you're able to update one of them or perform maintenance without affecting the other.