A subdomain is a website address, that is under a main, or a root domain name. One example is something.domain.com, where domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own website and records i.e. apart from employing the domain name as part of the whole web address, a subdomain can be fully independent and even hosted by a different company. Using a subdomain could be very handy if you want to have different sites that are connected in some way. For example, you may have an online store under the main domain plus a forum under a subdomain so as to supply means for your customers to ask questions and / or share opinions concerning your products. The advantage of using a subdomain instead of a subfolder is that the sites are going to be separated, so that you can update one of them or perform maintenance without affecting the other.