A subdomain is a part of a web address which is under the main domain, 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 only "example.com". Every subdomain can have its own web site and records and can even be hosted with a different company if you want to use a certain feature that's not provided by your current service provider. One example for using a subdomain is if you have a business website as well as an online store under a subdomain where clients can purchase your products. Additionally, you can have a forum in which they can comment on the products and by using subdomains instead of subfolders you are going to avoid any chance of all sites going down when you perform maintenance, or update one of the site scripts. Keeping the websites separated is less risky in case there is a script security breach.