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.
Another feature of XNA 2.0 is a tool for converting between Windows and XBox projects. While it may not eliminate the need to keep separate project files for the two platforms, it should at least make multi-targeting a bit less painful.
Speaking of projects, XNA 2.0 offers a new project type called "integrated content project". This allows developers and graphic artists to separate the game assets from the rest of the runtime and has support for the XNA Framework Content Pipeline.
Several APIs have been added or enhanced. For example, networking support has been added to allow for multiplayer games. This even supports games in which some players are on Windows while others are using an XBox.