A subdomain is part of a web address that is under the main domain name, for example name.example.com. Actually, even in www.example.com the "www" part is a subdomain because the fully qualified domain name is just "example.com". Every subdomain can have its own web site and records and can also be hosted using a different company if you need to use a specific feature which isn't provided by your current provider. An 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. In addition, you can have a forum where they can talk about the products and by using subdomains as an alternative to subfolders you'll avoid any probability of all sites going down when you perform maintenance, or update one of the website scripts. Keeping the websites separated is more secure in case there is a script security breach.