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.

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.

sometimes it's needed to check whether there are multiple same
modules in the current page or not to keep one version of the
module or some other checks.

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 -

if you are participating in a project of a
course,assignment,others on c#.net or asp.net
or may be others then you should follow the
steps for finishing the project successfully.

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).