What is a runtime host in ASP.Net?

.Net framework supports different type of applications like Web, windows, console etc,. Each type of application needs a runtime host to start it. This runtime host loads the runtime into a process, creates the application with in the process and loads the application code into the process. Runtime hosts included in .Net framework are
ASP.Net: It loads the runtime that can handle a web request into the process. ASP.NET also creates an application domain for each Web application that will run on a Web server.

Microsoft Internet Explorer: It creates an application domain to run managed controls.
Shell executables: When ever a runtime executable is launched from the shell, this executable invokes the corresponding runtime host.