A subdomain is a part of a web address that's under the main domain name, for instance name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every subdomain could have its own web site and records and can even be hosted with a different provider if you want to use a specific feature that's not offered by your current provider. One example for using a subdomain is if you have a business site as well as an online store under a subdomain where clients can acquire your products. In addition, you can have a forum in which they can talk about 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 also less risky in the event of a script security breach.