The XNA framework is Microsoft's new platform to allow people to develop games simultaneously on both Windows and the Xbox 360. It provides an easy to use content pipeline and a high level API making porting games from one system to another simple.
To share content between a PC and an Xbox 360 users must be part of a 'Creators Club'. Games will be sent to the console using an internet connection.
Whilst XNA is in beta the Creators Club is not open to the public due to Microsoft's reluctance to allow beta software on the Xbox 360.
XNA Game Studio Express
Game Studio Express is based on Microsoft's Visual C# Express 2005. It is aimed at students and hobbyists and is available for free download.
Games developed for the Xbox using Game Studio Express are limited to non commercial use, however games developed for Windows may be commercial.
XNA Game Studio Express is currently available in beta form for Windows XP; It is currently not supported in Vista.
XNA Game Studio Professional
Due for release in 2007, Game Studio Professional will allow the development of commercial Xbox 360 games.