A subdomain is part of a web address which is under the main domain name, for instance name.example.com. From a technical perspective, even in www.example.com the "www" element is a subdomain given that the fully qualified domain name is just "example.com". Each subdomain can have its own web site and records and can also be hosted using a different provider if you need to use some feature that isn't provided by your current provider. An example for using a subdomain is if you have a business website and an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum in which they can discuss the products and by employing subdomains instead of subfolders you will avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping your websites separated is more secure in case there is a script security breach.