A subdomain is a part of a web address that's under the main domain name, such as name.example.com. Technically, even in www.example.com the "www" element is a subdomain since the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can even be hosted using a different company if you need to use a specific feature that is not offered by your current service provider. A good example for using a subdomain is if you have a company site and an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can talk about the products and by using subdomains instead of subfolders you will avoid any probability of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also less risky in case of a script security breach.