Microsoft XNA is a set of tools with a managed runtime environment provided by Microsoft that facilitates video game development and management. XNA attempts to free game developers from writing "repetitive boilerplate code" and to bring different aspects of game production into a single system.
XNA is a very useful game development system, and though I personally have never used it, I have played many good indie games for the Xbox 360 that utilises this system. K. Lim.
(Please note that I am a friend of the owner of this account, and my opinions do not necessarily reflect his views.)