The Global.asax file, which is derived from the Http Application class,
maintains a pool of Http Application objects, and assigns them to applications
as needed.
The Global.asax file contains the following events:
Application_Init: Fired when an application initializes or is first called.
It is invoked for all Http Application object instances.
Application Error: Fired when an unhandled exception is encountered within the
application.
Application Start: Fired when the first instance of the Http Application
class is created. It allows you to create objects that are accessible by all
Http Application instances.
Application End: Fired when the last instance of an Http Application class is
destroyed. It is fired only once during an application's lifetime.
Session Start: Fired when a new user visits the application Web site.
Session End: Fired when a user's session times out, ends, or they leave the
application Web site.
See the following ASP.NET
interview questions video on importance of session and viewstate: -