A virtual directory is a link to an existing physical directory that is
present on the server’s hard disk. Virtual directories can have a number
of specific settings like custom ASP.NET configuration, access
permissions, and protection with a password.
Because any virtual directory can have its own settings, including
customized ASP.NET configuration, virtual directories are very useful in
setting up your web applications, especially those written in ASP.NET.
For example, if you have three web applications that use ASP.NET version
2.0, and you need to install one web application that uses ASP.NET
version 4.0, you can create a virtual directory for the ASP.NET 4.0
application, configure ASP.NET settings for this directory so as to
enable support for version 4.0, and then successfully install the
required application.
Virtual directories can also be used as aliases. For example, you have a
web application installed on your domain “example.com” in the physical
directory “/my_data/web_apps/forum". To access this web application,
users need to type “example.com/my_data/web_apps/forum", which is
hard to remember and too long to type. You can create virtual directory
called forum in the root of your virtual host, and link this virtual
directory to “/my_data/web_apps/forum", so the users who want to
access the web application will need to type “example.com/forum",
which is much shorter and easier to remember.