A subdomain is part of a web address that's under the main domain, for example name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain given that the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can even be hosted through a different provider if you wish to use some feature which isn't provided by your current provider. An illustration for using a subdomain is if you have a business site along with an online store under a subdomain where clients can acquire your products. You can also have a forum in which they can discuss the products and by using subdomains instead of subfolders you will avoid any risk of all sites going down if you perform maintenance, or update one of the site scripts. Keeping the sites separated is also less risky in case of a script security breach.