Global.asax 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.
There is a good list of events that are fired but following are few of the important events in Global.asax:
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 startsApplication_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 time out.