A game engine is a software system designed for the creation and development of video games. The core functions provided by a game engine usually include; a rendering engine for both 2D and 3D and physics engine for collision detection response, sound, scripting, animation, artificial intelligence (A.I) and several others.
Due to their nature game engines are constantly evolving, adapting to new requirements and incorporating new technologies. ‘Havok’ is a Cross-platform engine based around physics and is found in many of today's games. However, a lot of the most recent big budget titles seem to favour their own engine. I find it quite interesting that the games ‘Farcry’ and is sequel ‘Farcry 2’ use completely different engines, the ‘CryEngine’ and the ‘Dunia Engine’ respectively. It must have come at a large cost to the developers so presumably they intend for it to be used in other games and sold on to third parties.