-> is basically ASP.Net application file. It’s a place to write code for Application-level events such as Application start, Application end, Session start and end, Application error etc. raised by ASP.Net or by HTTP Modules.

Application_Init          – occurs in case of application initialization for the very first time.
 Application_Start       – fires on application start.
 Session_Start                – fires when a new user session starts.
Application_Error       – occurs in case of an unhandled exception generated from application.
Session_End                   – fires when user session ends.
Application_End          – fires when application ends or times out.