Master pages in ASP.NET allow you to create a consistent layout and standard behavior for pages in an application by defining the look and feel in a single master page that other content pages can be bound to. A master page consists of the master page file that defines placeholders and the content pages that populate those placeholders, allowing common features to be set on the master page while individual content is added on each content page. To create a master page, you first add the master page file and set the layout and common features, then add content pages to fill the placeholders.