Application state was covered in Chapter 4, which addressed the Application property of the ASP.NET Page object. I explained that the Application property is really an instance of the HttpApplicationState object and that you can initialize an Application variable in the Application_OnStart event that is contained in the Global.asax file located in the root of your application.
First you must remember that Application variables are just like Session variables, except that they are shared across all users. A Session variable holds information that is unique to that user. Application variables hold information that is common to all users.