ASP.NET Page Life Cycle

Each request for a Microsoft® ASP.NET page that hits Microsoft® Internet Information Services (IIS) is handed over to the ASP.NET HTTP pipeline. The HTTP pipeline is a chain of managed objects that sequentially process the request and make the transition from a URL to plain HTML text happen. The entry point of the HTTP pipeline is the HttpRuntime class. The ASP.NET infrastructure creates one instance of this class per each AppDomain hosted within the worker process...

For a detailed scenario of Page Life Cycle, just go through
http://msdn2.microsoft.com/en-us/library/aa479007.aspx