"ASP.NET 2.0 introduces a new concept known as Master Pages, in which you create a common base master file that provides a consistent layout for multiple pages in your application. To create a Master Page, you identify common appearance and behavior factors for the pages in your application, and move those to a master page. In the master page, you add placeholders called ContentPlaceHolders where the content (child) pages will insert their custom content. When users request the content pages, ASP.NET merges the output of the content pages with the output of the master page, resulting in a page that combines the master page layout with the output of the content page."
This comes with Visual Studio.NET Whidbey, which will be released later this year. It’s hard to believe it took Microsoft so long to create a templating system, but better late than never. Here’s advice on how to do it in the meantime (i.e. with .Net 1.x).