Master page concept was introduced in asp.net 2.0 first. It is a very nice and famous feature in asp.net. Now a day’s most of the site developed in asp.net is using this nice feature. This is great news for asp.net developer that VS 2008/ dot net 3.5 gives support for nested master page.
In your web project add a master page as root master page and add master pages under that page. These child master pages will also have .master extension. Child master pages contain content controls that are mapped in the content placeholders of its parent master page. The child master pages also have its own content placeholder to show content of its child pages.
Plese read this article to get an example and complete idea on it.