A subdomain is part of a web address that is under the main domain, for example name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is only "example.com". Every single subdomain could have its own web site and records and can also be hosted with a different provider if you want to use a specific feature that is not offered by your current provider. A good 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 where they can discuss the products and by using subdomains rather than subfolders you will avoid any chance of all sites going down when you perform maintenance, or update one of the website scripts. Keeping your sites separated is also more secure in the event of a script security breach.