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" element is a subdomain since the fully qualified domain name is just "example.com". Each subdomain could have its own web site and records and can also be hosted with a different provider if you wish to use a certain feature which is not provided by your current service provider. An example for using a subdomain is if you have a business website along with an online store under a subdomain where clients can buy your products. Additionally, you can have a forum where they can talk about the products and by employing subdomains rather than subfolders you are going to avoid any risk of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also more secure in the event of a script security breach.