What is life cycle in Asp.Net?

There are quite a few stages in an ASP.NET page's life cycle. There's an excellent article from Microsoft on MSDN with information on all the stages. This article lists both the general page life cycle stages that a user goes through and the life cycle events that trigger in the back end.

Article URL:

ASP.NET Page Life Cycle Overview

If the question is more broader for ASP.NET complete life cycle , 

I am giving few kewords.

You can check Machine.config, IIS lifecycle, that is the first points wherein the server determines how to handle the incoming request. 

There are IIS pipeline events ,HttpApplication,HttpContext , Global.asax events (if subscribed) HttpRequest and HttpResponse are created.

HttpModules and HttpHandlers which get triggered based on how they are declared.

After all these the request comes to ASP.NET Page Life cycle , there you have the Master Pages , ASPX Pages, User Controls (There are too many events and sometimes too confusing which trigger when you have multple asp.net controls that can trigger postback) and finally again some HttpModules , Global.asax (if subscribed).

Sounds a lot of overwork , hence MVC :)