ASP.NET MVC 3: Layouts and Sections with Razor

“In today’s post I’m going to go into more details about how Layout pages work with Razor.  In particular, I’m going to cover how you can have multiple, non-contiguous, replaceable “sections” within a layout file – and enable views based on layouts to optionally “fill in” these different sections at runtime.  The Razor syntax for doing this is clean and concise.

I’ll also show how you can dynamically check at runtime whether a particular layout section has been defined, and how you can provide alternate content (or even an alternate layout) in the event that a section isn’t specified within a view template.  This provides a powerful and easy way to customize the UI of your site and make it clean and DRY from an implementation perspective”

http://weblogs.asp.net/scottgu/archive/2010/12/30/asp-net-mvc-3-layouts-and-sections-with-razor.aspx