A subdomain is a part of a web address which is under the main domain, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Every subdomain could have its own website and records and can even be hosted using a different company if you want to use a specific feature which isn't provided by your current service provider. An example for using a subdomain is if you have a company website along with an online store under a subdomain where clients can buy your products. In addition, you can have a forum where they can comment on the products and by using subdomains as opposed to subfolders you are going to avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is also more secure in the event of a script security breach.