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. We 
will be covering both here.

it contains 7 events....

1.Application start
2.Application End
3.Session on start
4.Session on end
5.Session on Request
6.Application on Begining request
7.Application on Error

it is a collection of event handler

The Global.asax file (also known as the ASP.NET application 
file) is an optional file that is located in the 
application's root directory and is the ASP.NET counterpart 
of the Global.asa of ASP. This file exposes the application 
and session level events in ASP.NET and provides a gateway 
to all the application and the session level events in 
ASP.NET. This file can be used to implement the important 
application and session level events such as 
Application_Start, Application_End, Session_Start, 
Session_End, etc. This article provides an overview of the 
Global.asax file, the events stored in this file and how we 
can perform application wide tasks with the help of this 
file.

Global.asax is a text file ,it is use to define the global 
variable.