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. The XNA toolset was announced March 24, 2004, at the Game Developers Conference in San Jose, California. A first Community Technology Preview of XNA Build was released on March 14, 2006. XNA Game Studio 2.0 was released in December 2007, followed by XNA Game Studio 3.0 on October 30, 2008. XNA Game Studio 4.0 was released on September 16, 2010 along with the Windows Phone Development Tools.
XNA currently encompasses Microsoft's entire Game Development Sections, including the standard Xbox Development Kit and XNA Game Studio.
The name "XNA" originated out of the project's development name, Xbox New Architecture. Instead of being released under the Xbox name, the Xbox 360 was released (2005), and XNA came to stand for "XNA's Not Acronymed", which is true because XNA is an initialism, not an acronym.
Read more about Microsoft XNA: License Agreement, XNA Indie Games, Alternative Implementations, Partial List of Games That Use XNA