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 given that the fully qualified domain name is just "example.com". Each subdomain could have its own website and records and can even be hosted using a different provider if you wish to use a certain feature which isn't provided by your current provider. One example for using a subdomain is if you have a business website and an online store under a subdomain where customers can buy your products. Additionally, you can have a forum where they can discuss the products and by using subdomains instead of subfolders you will avoid any risk of all websites going down if you perform maintenance, or update one of the website scripts. Keeping the sites separated is less risky in the event of a script security breach.