What is master page in ASP.NET?

The master page defines placeholders for the content, which can be overridden by content pages. The output result is a combination of the master page and the content page . This improves the maintainability of your site and avoids unnecessary duplication of code for shared site structure or behaviour. A single master page defines the look and feel and standard behaviour that you want for all of the pages (or a group of pages) in your application. Defining a Master page is similar to defining a normal page, but saving a Master page is different from saving a normal page. You must save the Master pages by using the .master file extension. Master pages are completely transparent to end users and permit developers to create Web sites where pages share a common layout . The page layout for Master Pages is easy to create, simple to maintain, and simple to assign to a Web application. The master page is identified by a special @ Master directive that replaces the @ Page directive that is used for ordinary .aspx pages. example
< %@ Master Language="C#" % >
Whenever a master page's layout or formatting is changed, all of its content pages output is likewise immediately updated, which makes applying site-wide appearance changes as easy as updating and deploying a single file .