A subdomain is part of a web address that's under the main domain, for example name.example.com. Technically, 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 site and records and can also be hosted using a different provider if you need to use some feature that's not offered by your current service provider. A good example for using a subdomain is if you have a business website as well as an online store under a subdomain where customers can purchase your products. You can also have a forum where they can talk about the products and by using subdomains as an alternative to 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 websites separated is also more secure in the event of a script security breach.