Web application

http://msdn.microsoft.com/en-us/library/bb892189%28v=office.12%29.aspx

Once a Web application is created, it is no longer necessary to touch the file system or IIS metabase of the front-end Web server when creating, updating, and deleting sites or site collections.

 

web application-->并不存放在File System

site collection-->存放在content database