A subdomain is a part of a web address that is under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is only "example.com". Each subdomain can have its own website and records and can even be hosted with a different provider if you wish to use some feature that isn't offered by your current service provider. One 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 employing subdomains as opposed to subfolders you are going to avoid any risk of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also more secure in the case of a script security breach.