A subdomain is a part of a web address that is under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can even be hosted using a different provider if you need to use a specific feature which is not provided by your current service provider. An illustration for using a subdomain is if you have a business site and an online store under a subdomain where clients can purchase your products. In addition, you can have a forum in which they can discuss the products and by using subdomains as an alternative to subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is also less risky in case of a script security breach.