Blender is an open source 3D modelling software with built-in game engine, Bullet physics engine and fluid animation engine. Questions involving issues with Blender itself should be posted on the relevant Stack Exchange site, Blender.SE.
Blender is extremely popular among enthusiasts and has started to be taken seriously in production too. It has been used in the movie 2012 and numerous commercials. It is now maintained by Blender Foundation, which is a non-profit organization. Blender Foundation frequently undertakes development of challenging and high quality open source movies which allows accelerated development of Blender.
This tag should contain questions relating to using Blender in the context of game development, like using exported models. Questions regarding the program and its use should be asked on Blender.SE, the official Stack Exchange site for the Program.