a site contains one or more applications, an application contains one or more virtual directories, and a virtual directory maps to a physical directory on a computer.
A site is a container for applications and virtual directories, and you can access it through one or more unique bindings.
An application is a group of files that delivers content or provides services over protocols, such as HTTP. When you create an application in IIS, the application's path becomes part of the site's URL.
each site must have an application which is named the root application
A virtual directory is a directory name (also referred to as path) that you specify in IIS and map to a physical directory on a local or remote server.