system.web.ui.page

Master Page means in name itself it lies.
MasterPage is a Common Page for all the SubPages.
Without  going to SubPages Separately,We can goto SubPages 
directly by clicking on Master Page.
It reduces the Work.
This Current Page is an best Example to understand about 
MasterPage.
In MasterPage we most prefer treeview to go for subpages.

Master pages let you make a consistent layout for your 
application, you can make one master page that holds the 
layout/look & feel and common functionality of your whole 
application and upon this master page, you can build all 
the other pages, we call these pages Content Pages. So 
simply you can build your master page and then build 
content pages, and while creating the content pages you 
bind them to the master page you have created before, those 
two pages are merged at runtime to give you the rendered 
page

master page is the way to centralize the common 
functionality of pages. common functionalities like: 
header, footer of page, logo or the site name.
All controls which are placed in the content page or the 
child page are within the ContentPlaceHolder which is a 
child control of the master page.
now master page is merged and treated like a control in the 
content page.

Master pages are used to giving common layout for our 
applications.
we never change our header,footer.
we should change body using content place holder.
this is very useful for reuseful components of our web page