The master pages in ASP.NET basically allow you to create a consistent layout and look and feel for the pages in your website.

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. We 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..