jack Said..
The Global.asax file is in the root (in the global area) of your application directory. Visual Studio .NET automatically inserts it in all new ASP.NET projects but actually it's an optional file. The ASP.NET page framework recognizes automatically any changes that are made to the Global.asax file.
The Global.asax file is used to handle application-level and session-level events. 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.
while adding Global.asax file to our application bydefault it contains five methods,
Those methods are:
1.Application_Start.
2.Application_End.
3.Session_Start.
4.Session_End.
5.Application_Error.