A subdomain is 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" element is a subdomain because the fully qualified domain name is only "example.com". Each subdomain could have its own web site and records and can even be hosted with a different company if you wish to use some feature which isn't provided by your current service provider. An example for using a subdomain is if you have a company website and an online store under a subdomain where clients can acquire your products. In addition, you can have a forum where they can talk about the products and by using subdomains instead of subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is also more secure in the case of a script security breach.