A subdomain is a part of a web address which is under the main domain name, such as name.example.com. Actually, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Every subdomain can have its own site and records and can also be hosted through a different company if you need to use a feature which is not provided by your current service provider. A good example for using a subdomain is if you have a company website and an online store under a subdomain where customers can buy your products. Additionally, you can have a forum in which they can discuss the products and by employing subdomains rather than subfolders you are going to avoid any chance of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the sites separated is more secure in the event of a script security breach.