all asp.net handlers implement the interface of IHttpHandler located
in the namespace of System.Web.ISAPI(Internet server application
programming interface) extensions are somehow analogous to Handlers.

master pages creates a common look in your application.
if we think about the header and footer in our pages,we
create these header and footer in every aspx pages.this
method has disadvantages.if there were any custom page
which dynamically adds header and footer then would be
good.

The great control in the asp.net ajax is the update
panel control.it can be added to almost every
asp.net page almost magically.this control when
used with a ScriptManager permits a programmer to
use partial rendering(a technique by which many
sections of page updated partially without reloading
the entire page).

in asp.net application or website </appSettings>
section in web.config is used to store application
setting to be used.you can keep settings in this
section as -

it is possible to run Application.Run twice in csharp
application.the following example shows that.it's a
example to create login to access the application.