Session Vs Application Variable

Session Variables are those variable which only available to a single user. They are used to store/display single user information on website.

Application Variables are common to all user. These variables are used to configure application settings/other stuff.

Please read article, if you want to know more about Session.

What Causes Application Restart

ASP.net run-time environment implements a good deal of checks and automatically restarts an application if any of the following scenarios occur:

• The maximum limit of dynamic page compilations is reached.
• The physical path of the Web application has changed, or any directory under the Web application folder is renamed.
• Changes occurred in global.asax, machine.config or web.config in the application root, or in the Bin directory or any of its subdirectories.
• Changes occurred in the code-access security policy file, if one exists.
• Too many files are changed in one of the content directories. (Typically, this happens if files are generated on the fly when requested.)
• Changes occurred to settings that control the restart/shutdown of the ASP.NET worker process. These settings are read from machine.config if you don't use Windows 2003 Server with the IIS 6.0 process model. If you're talking full advantage of IIS 6.0, an application is restarted if you modify properties in the Application Pools node of the IIS manager.