Global.asax is a file that resides in the root directory 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 can come from one of 
two places - The HTTPApplication object and any HTTPModule 
object that is specified in web.confg or machine.config


Regards

sridhara krishna bodavula

it allows to execute application level events and set 
application level variables.

The Global.asax file, sometimes called the ASP.NET 
application file, provides a way to respond to application 
or module level events in one central location. You can use 
this file to implement application security, as well as 
other tasks.
The Global.asax file is in the root application directory. 
While Visual Studio .NET automatically inserts it in all 
new ASP.NET projects, it's actually an optional file. It's 
okay to delete it—if you aren't using it. The .asax file 
extension signals that it's an application file rather than 
an ASP.NET file that uses aspx.
The Global.asax file is configured so that any direct HTTP 
request (via URL) is rejected automatically, so users 
cannot download or view its contents. The ASP.NET page 
framework recognizes automatically any changes that are 
made to the Global.asax file. The framework reboots the 
application, which includes closing all browser sessions, 
flushes all state information, and restarts the application 
domain.