A subdomain is part of a web address which is under the main domain name, such as 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 single subdomain can have its own website and records and can also be hosted with a different company if you want to use a specific feature which is not provided by your current service provider. A good example for using a subdomain is if you have a company website and an online store under a subdomain where clients can acquire your products. Additionally, you can have a forum where they can comment on the products and by using subdomains rather than subfolders you will avoid any chance of all websites going down if you perform maintenance, or update one of the site scripts. Keeping your websites separated is less risky in case of a script security breach.