A subdomain is part of a web address that is under the main domain, such as 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 just "example.com". Each subdomain could have its own site and records and can also be hosted with a different company if you want to use some feature that's not offered by your current provider. An illustration for using a subdomain is if you have a company site along with an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum in which they can comment on the products and by employing subdomains rather than subfolders you are going to avoid any chance of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is also less risky in the case of a script security breach.