This package is the add-on for Sisual Studio 2008 that provides:

• Visual Basic and C# Project templates
• Intellisense and code generators for XAML
• Debugging of Silverlight applications
• Web reference support
• Integration with Expression Blend

Same thing you had before for Visual Studio 2008 Beta 2. Only for Vista or Windows XP. Requires VS2008 Standard or above. That means, this release will not work with the Express edition. 

That means is another step to install VS2008 release from the previous mention

Cheers

Al