Tabs provide a simple way of presenting sets of controls or documents on separate, labeled pages. They are quite popular and most commonly associated with typical property windows. On the Web, they are also very popular as a means for content organization.
As a very general guideline, tabs are used when you are dealing with too much information on one page (it doesn't matter if it is a web or desktop application), which basically creates confusion making the specific content difficult to find and focus on.
Start your Expression Blend 4 and then select New project... From the dialog that appears select Silverlight and then Silverlight Application. Make sure that Language is set to C# and Version is 4.0. At the end hit OK.