XNA Game studio is a game development toolkit that supports both Windows and the XBox 360. The initial edition was targeted directly at hobbyists and released as an extension to C# Express Edition.
The beta for XNA 2.0 brings the toolkit closer to the world of professional developers. The number one feature is that it now supports all versions of Visual Studio 2005. This unlocks tools such as integrated source control and testing frameworks. Multi-threaded debugging is enhanced and developers can use any VS Add-In they may have.