A subdomain is a part of a web address that is under the main domain name, for instance name.example.com. Actually, even in www.example.com the "www" part is a subdomain since the fully qualified domain name is just "example.com". Every subdomain could have its own web site and records and can also be hosted through a different provider if you wish to use a specific feature that's not offered by your current service provider. A good example for using a subdomain is if you have a company site as well as an online store under a subdomain where customers can acquire your products. Additionally, you can have a forum where they can comment on the products and by using subdomains instead of subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping your websites separated is also less risky in case there is a script security breach.