A subdomain is part of a web address that's under the main domain name, for example name.example.com. From a technical perspective, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is only "example.com". Every single subdomain can have its own web site and records and can also be hosted through a different company if you wish 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 website along with an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum in which they can comment on the products and by using subdomains as an alternative to subfolders you are going to avoid any risk of all websites going down when you perform maintenance, or update one of the site scripts. Keeping your sites separated is less risky in the case of a script security breach.