A subdomain is a 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 since the fully qualified domain name is only "example.com". Each subdomain could have its own site and records and can also be hosted using a different provider if you wish to use a feature that is not provided by your current provider. An example for using a subdomain is if you have a business website and an online store under a subdomain where clients can buy your products. Additionally, you can have a forum where they can comment on the products and by using subdomains as an alternative to subfolders you will avoid any probability of all websites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is also more secure in the event of a script security breach.