To implement application and session level events, the 
Global.asax is used.

The Global.asax file, also known as the ASP.NET application
file, is an optional file that contains code for responding
to application-level events raised by ASP.NET or by HTTP
modules. 

 For More information Visit..
http://msdn.microsoft.com/library/default.asp?url=/library/en-
us/cpgenref/html/cpconGlobalasaxSyntax.asp

Global.asax file is used for storing values at session 
level and Application level.

Global.asax is a file, as the name suggests that resides in 
the root directory (in the global area) of your 
application. It is inaccessible over the web but is used by 
the ASP.NET application if it is there. It is a collection 
of event handlers that you can use to change and set 
settings in your site. The events may occur from one of two 
places - The HTTPApplication object and any HTTPModule 
object that is specified in web.config or machine.config.

Global.asax file is used for storing values at session 
level and Application level.

It allows you to execute application level events and 
settings application level variables