This document provides information about creating and using master pages in ASP.NET. It defines a master page as a page that stores global elements that occur on every content page, such as headers, footers, menus. It allows for consistent styling across pages. The document demonstrates how to create a master page file with the .master extension, add placeholders for content, and then create content pages that inherit from the master page to populate those placeholders. Examples are provided of the code needed in both the master page and content pages.