A master page is a template that you can use to define the overall look and feel of your ASP.NET Web applications. In previous versions of ASP.NET, you were forced to embed user controls in every page to get consistency. Master pages now make it much easier to deliver a consistent look and feel, without the need to embed user controls across all of your pages.
Here is an article from ftp online: Create Master Pages in ASP.NET 2.0.