You can design a master page which would be the frame look  
of your website. Later you can create pages under this 
Master page ..so that what ever pages your oppening  will 
be in this master page . 

Eg : You create a master page  and putting some table for 
header,then for left pane then for footer also . the middle 
portion will be the content of all your child(?) pages. 
Early ASP days we used FRAMES for this kind of DESIGN . 
This master page is good for a WEB SITE DESIGN (even for 
Good in INTRANET WEB APPLICATIONS)

ASP.NET master pages allow you to create a consistent 
layout for the pages in your application. A single master 
page defines the look and feel and standard behavior that 
you want for all of the pages (or a group of pages) in your 
application. 

You can then create individual content pages that contain 
the content you want to display. When users request the 
content pages, they merge with the master page to produce 
output that combines the layout of the master page with the 
content from the content page.

Master page tools is a new tools comes with ASP.Net 
2.0.This tools is used for creating similar page layout and 
style for every content page(this page creted like child 
page of master page) in our web application.